x86/mce: adjust comment of callback register functions

x86/mce: adjust comment of callback register functions

c/s e966818264908e842e2847f579ca4d94e586eaac added
mce_need_clearbank_register below the comment of
x86_mce_callback_register(). This commit (1) adjusts the first
paragraph of comment to be a general statement of all callback
register functions, and (2) moves the second paragraph to the
front of x86_mce_callback_register().

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>