southbridge/sis: Spelling/comment fixes

Change-Id: I6a0f5406fb3bc3e8aa3a1111b1d702f530c9329b
Signed-off-by: Martin Roth <martin.roth@se-eng.com>
Reviewed-on: http://review.coreboot.org/7733
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>