NFC: port100: Don't send a new command if one is still pending

This patch ensures that a command is not still in process before sending
a new one to the device. This can happen when neard is in constant
polling mode: the configure_hw command can be sent when neard restarts
polling after a LLCP SYMM timeout but before the device has returned in
timeout from the last DEP frame sent.

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