[media] cx18: support big-endian systems

base_addr has type resource_size_t, which may be 64 bits.

Also fix a few endian issues related to mailboxes and firmware loading.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>