bridge: netfilter: use PTR_RET instead of IS_ERR + PTR_ERR

This uses PTR_RET instead of IS_ERR and PTR_ERR in order to increase
readability.

Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>