net: lan78xx: Fix to write to OTP(One Time Programmable) per magic number.

This patch fixes a bug writing to EEPROM in lan78xx_ethtool_set_eeprom()
when asked to write to OTP.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>