Bluetooth: L2CAP: Restore credits after receiving an SDU

This simplify the logic of restoring the credits after each SDU instead
of using an arbitrary threshold which was not configurable per channel.

Because the credits are restored only when the full SDU has been
reassembled it means the channels needs to be set up with enough for
the configured MTU otherwise there is a risk of the channel to run out
of RX credits before the packet is fully reassembled, because of such
corner case the code will now warn if a channel is setup with not enough
init_credits.

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