asix: Rework reading from EEPROM

The current code for reading the EEPROM via ethtool in the asix
driver has a few issues. It cannot handle odd length values
(accesses must be aligned at 16 bit boundaries) and interprets the
offset provided by ethtool as 16 bit word offset instead as byte offset.

The new code for asix_get_eeprom() introduced by this patch is
modeled after the code in
drivers/net/ethernet/atheros/atl1e/atl1e_ethtool.c
and provides read access to the entire EEPROM with arbitrary
offsets and lengths.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>