x86/mce: Cleanup __mc_scan_banks()

Correct comments, improve readability, simplify.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180622095428.626-7-bp@alien8.de