memory: Fix memory_region_wrong_endianness()

Since commit be675c972088eba210e18dc125613e9f205a6bfb (memory: move
endianness compensation to memory core) it was checking for
TARGET_BIG_ENDIAN instead of TARGET_WORDS_BIGENDIAN, thereby not
swapping correctly for Big Endian targets.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Avi Kivity <avi@redhat.com>
