nfc: pn533: Constify serdev_device_ops

The only usage of pn532_serdev_ops is to pass its address to
serdev_device_set_client_ops(), which takes a pointer to const
serdev_device_ops as argument. Make it const to allow the compiler to
put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>