nfc: constify nfc_phy_ops

Neither the core nor the drivers modify the passed pointer to struct
nfc_phy_ops (consisting of function pointers), so make it a pointer
to const for correctness and safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>