[media] dib9000: fix return type in dib9000_mbx_send_attr()

dib9000_mbx_send_attr() returns an int.  It doesn't work to save
negative error codes in an unsigned char, so I've made "ret" an int
type.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>