oct612x: Use ccflags-y instead of EXTRA_CFLAGS.

[InterLinked1]

ccflags-y was added to the kernel back in 2007, in commit f77bf01425b11947eeb3b5b54. Recent kernel commit
dbd83ea09699390892e5efecddd74ae43a00f071 has now completely removed the deprecated EXTRA_CFLAGS.

Comments in Kbuild and the Makefile for the oct612x library were added back when it was created in 2013 in commit f65299e to use the newer ccflags-y based on the kernel version, but the change was never made to conditionally move away from the EXTRA_CFLAGS.

Now that the older way no longer exists, always use ccflags-y.

Resolves: #76

[InterLinked1]

@push143smart @duncanpatterson It's now been another half a year now with zero maintenance or attention from the DAHDI team. We have been asking nicely and patiently for years now (literally). I'm going to again ask that all the fixes sitting here for critical compilation errors be merged and a new release cut. There are a bunch of fixes sitting that are critically needed. For your reference:

        # Merged in master, but not yet in a current release
	dahlin_apply_commit "d7bbc8a96fe767bc4eee15dd43170f298282a4c3" # RHEL fixes for const struct device *dev
	dahlin_apply_commit "d932d9fbc8b3559829a76fffcedceb78d1fc1887" # dahdi_spantype fix
	dahlin_apply_pr 32 # PR 32: xpp: Fix 32-bit builds
	dahlin_apply_pr 57 # PR 57: RHEL build fixes
	dahlin_apply_pr 58 # PR 58: non-RHEL build fixes for older kernels
	dahlin_apply_pr 60 # PR 60: Fix old style declaration error on newer systems
	dahlin_apply_pr 62 # PR 62: Rename MAX to MAX_ATTEMPTS to avoid macro redefinition
	dahlin_apply_pr 64 # PR 64: More struct device to const struct device
	dahlin_apply_pr 66 # PR 66: Add braces around empty if body
	dahlin_apply_pr 69 # PR 69: DEFINE_SEMAPHORE for RHEL

	# Not yet merged
	dahlin_apply_pr 77 # EXTRA_CFLAGS removal
	dahlin_apply_pr 92 # del_timer_sync wrapper
	dahlin_apply_pr 96 # from_timer renamed to timer_container_of
	dahlin_apply_pr 98 # hrtimer_init changed to hrtimer_setup
	dahlin_apply_pr 99 # use module_init/module_exit instead of init_module/cleanup_module

Take a look through the issues, there are a ton of duplicate issues open, including for things that have already been merged, due to the releases being stale. I have limited ability to triage everything going on here (which I have been, since nobody "official" is showing any face here).

Sincerely, an actual DAHDI maintainer