net: phy: avoid NPE if read_page/write_page callbacks are not available

Currently there's a bug in the module subsystem [0] preventing load of
the PHY driver module on certain systems (as one symptom).
This results in a NPE on such systems for the following reason:
Instead of the correct PHY driver the genphy driver is loaded that
doesn't implement the read_page/write_page callbacks. Every call to
phy_read_paged() et al will result in a NPE therefore.

In parallel to fixing the root cause we should make sure that this one
and maybe similar issues in other subsystems don't result in a NPE
in phylib. So let's check for the callbacks before using them and warn
once if they are not available.

[0] https://marc.info/?t=157072642100001&r=1&w=2



Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>