Make check_signature depend on CONFIG_HAS_IOMEM

This should avoid build problems on architectures without a "readb()",
that got bitten by check_signature() being uninlined.

Noted by Heiko Carstens.

Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>