staging: xgifb: fix colours on big-endian machines other than powerpc

XGI framebuffer supports big-endian machines, but it's currently
enabled based on __powerpc__ define (which is wrong, as powerpc can
be also little-endian now). Use __BIG_ENDIAN instead. This will fix
wrong colours on such machines.

Tested on parisc with XGI Z7.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>