set_antenna() is supported only when the device is not started in
mac80211 which translates to the firmware not being loaded in iwlwifi.

The tricky part is that iwlwifi populates the sband data during its boot
and doesn't touch this data afterwards, but if the antenna settings
forbid MIMO, we need to update the sband data.

Rework the nvm parsing code to allow to get an existing nvm_data and
modify the sband with additional constraints (tx / rx chains masks).
Suggested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230921110726.81d94d630c95.I9473da818cbeeb51b2f89dcc59b00019113e7f55@changeid
[add bugfix from Benjamin for iwl_mvm_get_valid_rx_ant()]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>