Bluetooth: Use NET_BUF_RX_COUNT if NET_L2_BT is enabled

If NET_L2_BT is enabled we need enough acl_in_pool needs to be big
enough to contain a full IP packet since that is no longer processed by
RX thread buffer would be queued to syswq to reassemble the SDU.

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