NFC: Use IDR library to assing NFC devices IDs

As a consequence the NFC device IDs won't be increasing all the time,
as IDR provides the first available ID.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/include/net/nfc/nfc.h b/include/net/nfc/nfc.h
index f05b106..fce80b2 100644
--- a/include/net/nfc/nfc.h
+++ b/include/net/nfc/nfc.h
@@ -95,7 +95,7 @@
 };
 
 struct nfc_dev {
-	unsigned int idx;
+	int idx;
 	u32 target_next_idx;
 	struct nfc_target *targets;
 	int n_targets;