Bluetooth: L2CAP: Prevent the channel to stall

If the remote stack is not able to fully utilize each segment it is
possible it would run out of credits before completing the SDU, these
changes detects if that would happen and attempt to restore enough
credits for the SDU to be received.

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