NFC: Fix target mode p2p link establishment

With commit e29a9e2a, we set the active_target pointer from
nfc_dep_link_is_up() in order to support the case where the target
detection and the DEP link setting are done atomically by the driver.
That can only happen in initiator mode, so we need to check for that
otherwise we fail to bring a p2p link in target mode.

Signed-off-by: Arron Wang <arron.wang@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>