usb: bluetooth: l2cap buffer size now configurable

Added check to see if CONFIG_BT_CTLR_TX_BUFFER_SIZE is defined,
and use that as the BT_L2CAP_MTU if that is the case. Otherwise,
use the previously hard coded MTU size (64). This enables users
to configure the MTU, enabling longer transfers.
The fix was inspired by how this is solved in
samples/bluetooth/hci_uart/src/main.c

Tested on nrf52840_pca10056.

Signed-off-by: Dennis Wildmark <dennis.wildmark@assaabloy.com>