wifi: mac80211: update the rx_chains after set_antenna()

rx_chains was set only upon registration and it we rely on it for the
active chains upon SMPS configuration after association.

When we use the set_antenna() API to limit the rx_chains from 2 to 1,
this caused issues with iwlwifi since we still had 2 active_chains
requested.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230920211508.2dde4da246b2.I904223c868c77cf2ba132a3088fe6506fcbb443b@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>