NFC: digital: Handle multiple SENSF_REQ frames

According to section 5.15.1.3 of the NFC Activity
Specification, multiple SENSF_REQ commands can be
received by a target before it receives an ATR_REQ
command.  To handle this, add a routine that checks
whether a SENSF_REQ or ATR_REQ has been recieved.
If its a SENSF_REQ, respond appropriately and
continue waiting for a ATR_REQ.  If its an ATR_REQ,
handle it as before.

CC: 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>