iwlagn: implement layout-agnostic EEPROM reading

The current EEPROM reading code has some layout
assumptions that now turned out to be false with
some newer versions of the EEPROM. Luckily, we
can avoid all such assumptions by using data in
the EEPROM itself, so implement using that.

However, for risk mitigation purposes, keep the
old reading code for current hardware for now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>