staging: speakup: speakup_dectlk.c: style fixes

* Clean this file based on reports from checkpatch.pl.
* Replace a function-like macro with an inline function.
* Remove an incorrect use of the volatile qualifier.  The
previously-volatile variable is now always protected by a spinlock.

Signed-off-by: Christopher Brannon <chris@the-brannons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>