Bluetooth: Fix missing channel unlock in l2cap_le_credits

In the error case where credits is greater than max_credits there
is a missing l2cap_chan_unlock before returning.

Signed-off-by: Martin Townsend <mtownsend1973@gmail.com>
Tested-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>