Bluetooth: L2CAP: Fix invalid access if a buffer cannot be allocated

Since bt_l2cap_create_pdu can return NULL when used under syswq context
the code should always check its result, this also changes the timeout
to have a specific value (RTX maximum timeout) so signalling PDUs still
wait for a buffer to become available.

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