ethtool: wire in generic SFP module access

If the device has a sfp bus attached, call its
sfp_get_module_eeprom_by_page() function, otherwise use the ethtool op
for the device. This follows how the IOCTL works.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>