Bluetooth: Check MTU value in l2cap_sock_setsockopt_old

If user tries to set an invalid MTU value, l2cap_sock_setsockopt_old
should return -EINVAL.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>