drivers/firmware/sigma.c needs MODULE_LICENSE

Fix module tainting message:

  sigma: module license 'unspecified' taints kernel.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>