nfc: pn533: Constify pn533_phy_ops

Neither the driver or the core modifies the pn533_phy_ops struct, so
make them const to allow the compiler to put the static structs in
read-only memory.

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