Bluetooth: L2CAP: Use NET_BUF_TX_COUNT as fragment count

When NET_L2_BT the memory pressure for fragments can be quite high
since that would be transfering IP packets which are considerable big
so this makes our frag_pool to be of the same size as NET_BUF_TX_COUNT.

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