x86_64: fix incorrect comments

Impact: cleanup

The comments which fxrstor_checking and fxsave_uset refer to is
now in fxsave. Change the comments appropriately.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Jiri Slaby <jirislaby@gmail.com>
LKML-Reference: <1239190320-23952-3-git-send-email-jirislaby@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>