net: sfp: avoid EEPROM read of absent SFP module

If SFP module is not present, it is sensible to fail sfp_module_eeprom()
and sfp_module_eeprom_by_page() early to avoid excessive I2C transfers
which are garanteed to fail.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>