nfc: pn532_uart: Make use of pn532 autopoll

This switches the pn532 UART phy driver from manually polling to the new
autopoll mechanism.

Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: David S. Miller <davem@davemloft.net>