x86/microcode/AMD: Clean up find_equiv_id()

No need to have it marked "inline" - let gcc decide. Also, shorten the
argument name and simplify while-test.

While at it, make it into a proper for-loop and simplify it even more,
as tglx suggests.

No functionality change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20170120202955.4091-5-bp@alien8.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>