CAPI: Clean up capi_open/release

Fold capidev_alloc and capidev_free into capi_open and capi_release -
there are no other users. Someone pushed a lock_kernel into capi_open.
Drop it, we don't need it. Also remove the useless test from open that
checks for private_data == NULL.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>