NFC: Add NFC_CMD_DEACTIVATE_TARGET support

Once an NFC target (i.e., a tag) is found, it remains active until
there is a failure reading or writing it (often caused by the target
moving out of range).  While the target is active, the NFC adapter
and antenna must remain powered.  This wastes power when the target
remains in range but the client application no longer cares whether
it is there or not.

To mitigate this, add a new netlink command that allows userspace
to deactivate an active target.  When issued, this command will cause
the NFC subsystem to act as though the target was moved out of range.
Once the command has been executed, the client application can power
off the NFC adapter to reduce power consumption.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/uapi/linux/nfc.h

@@ -89,6 +89,7 @@
  * @NFC_CMD_ACTIVATE_TARGET: Request NFC controller to reactivate target.
  * @NFC_CMD_VENDOR: Vendor specific command, to be implemented directly
  *	from the driver in order to support hardware specific operations.
+ * @NFC_CMD_DEACTIVATE_TARGET: Request NFC controller to deactivate target.
  */
 enum nfc_commands {
 	NFC_CMD_UNSPEC,
@@ -121,6 +122,7 @@ enum nfc_commands {
 	NFC_CMD_SE_IO,
 	NFC_CMD_ACTIVATE_TARGET,
 	NFC_CMD_VENDOR,
+	NFC_CMD_DEACTIVATE_TARGET,
 /* private: internal use only */
 	__NFC_CMD_AFTER_LAST
 };