NFC: digital: Add NFC-DEP Initiator-side ATN Support

When an NFC-DEP Initiator times out when waiting for
a DEP_RES from the Target, its supposed to send an
ATN to the Target.  The Target should respond to the
ATN with a similar ATN PDU and the Initiator can then
resend the last non-ATN PDU that it sent.  No more
than 'N(retry,atn)' are to be send where
2 <= 'N(retry,atn)' <= 5.  If the Initiator had just
sent a NACK PDU when the timeout occurred, it is to
continue sending NACKs until 'N(retry,nack)' NACKs
have been send.  This is described in section
14.12.5.6 of the NFC-DEP Digital Protocol Spec.

The digital layer's NFC-DEP code doesn't implement
this so add that support.

The value chosen for 'N(retry,atn)' is 2.

Reviewed-by: Thierry Escande <thierry.escande@linux.intel.com>
Tested-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/net/nfc/digital.h

@@ -233,6 +233,7 @@ struct nfc_digital_dev {
 	struct sk_buff *chaining_skb;
 	struct digital_data_exch *data_exch;
 
+	int atn_count;
 	int nack_count;
 
 	struct sk_buff *saved_skb;