Bluetooth: L2CAP: Derive rx credits from MTU and MPS

Give enough rx credits for a full packet instead of using an arbitrary
number which may not be enough depending on the MTU and MPS which can
cause interruptions while waiting for more credits, also remove
debugfs entry for l2cap_le_max_credits.

With these changes the credits are restored after each SDU is received
instead of using fixed threshold, this way it is garanteed that there
will always be enough credits to send a packet without waiting more
credits to arrive.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>