Bluetooth: L2CAP: Fix not checking for L2CAP_ECRED_CHAN_MAX

When receiving L2CAP_CREDIT_BASED_CONNECTION_REQ the remote may request
more channels than allowed so this checks if amount of channel surpasses
the maximum channels (5) and return an error.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>