net: bt: Fix not checking channel state

When checking if the L2CAP channel is connected it is not enough to check
if the conn member is set since the connection may still be pending.

Change-Id: I93a2328943b0ca0e42cdb5c525b30c7cdddd1c18
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>