NFC: llcp: Fix non blocking sockets connections

Without the new LLCP_CONNECTING state, non blocking sockets will be
woken up with a POLLHUP right after calling connect() because their
state is stuck at LLCP_CLOSED.
That prevents userspace from implementing any proper non blocking
socket based NFC p2p client.

Cc: stable@vger.kernel.org
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>