[media] mantis: cleanup NULL checking in mantis_ca_exit()

Smatch complainst that the call to mantis_evmgr_exit() dereferences "ca"
but then we check it for NULL on the next line.  I've moved the NULL
check forward to avoid that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>