ethtool: Split ethtool_get_eeprom() to allow for additional EEPROM accessors

We want to support reading module (SFP+, XFP, ...) EEPROMs as well as
NIC EEPROMs.  They will need a different command number and driver
operation, but the structure and arguments will be the same and so we
can share most of the code here.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>