iwlwifi: nvm: remove reading valid antennas from NVM

These values aren't used as the firmware values should be used,
so reading them is pointless and hides potential errors when
somebody uses these values. Leave them zero to make it clearer
that they can't be used. We can't remove the struct members as
the DVM driver does read the values from EEPROM/OTP and the
structure is shared between drivers.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>