NFC: pn533: improve cmd queue handling

Make sure cmd is set before a frame is passed to the transport layer for
sending. In addition pn533_send_async_complete checks if cmd is set before
accessing its members.

Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>

Rework a little bit changes in pn532_send_async_complete.

Signed-off-by: Andrey Rusalin <arusalin@dev.rtsoft.ru>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>