KSZ8851-SNL: Add ethtool support for EEPROM via eeprom_93cx6

Add ethtool EEPROM read/write support using the eeprom_93cx6
library instead of open-coding the functions.

Depends on eeprom_93cx6 driver getting EEPROM write support.

Change-Id: I06f3ab2b936244f658faae3ae8199a21de7b4d51
Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
[sboyd@codeaurora.org: Removed previous eeprom implementation]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>