NFC: Cache the core NFC active target pointer instead of its index

The NFC Core now caches the active nfc target pointer, thereby avoiding
the need to lookup the target table for each invocation of a driver ops.
Consequently, pn533, HCI and NCI now directly receive an nfc_target
pointer instead of a target index.

Cc: Ilan Elias <ilane@ti.com>
Signed-off-by: Eric Lapuyade <eric.lapuyade@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>