Bluetooth: l2cap: Fix initial credit calculaton for MTU < MPS

When required Rx MTU is less than configured Rx MPS, the
resultant initial credits was 0 which prevented any L2CAP
packet to be received.

Fixed by ceiling the initial credits count in the credits
calculation.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>