NFC: digital: Fix RTOX supervisor PDU handling

When the target needs more time to process the received PDU, it sends
Response Timeout Extension (RTOX) PDU.

When the initiator receives a RTOX PDU, it must reply with a RTOX PDU
and extends the current rwt value with the formula:
 rwt_int = rwt * rtox

This patch takes care of the rtox value passed by the target in the RTOX
PDU and extends the timeout for the next response accordingly.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>