epic100: Test __BIG_ENDIAN instead of (non-existent) CONFIG_BIG_ENDIAN

Probably no one has used this driver on big-endian systems, since it was
setting up descriptor swapping if CONFIG_BIG_ENDIAN is set, which it
never is, since that symbol is not mentioned anywhere else in the kernel
source.  Switch this test to a check for __BIG_ENDIAN so it has a chance
at working.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>