iwlwifi: mvm: remove rs FSM actions relevant only for 3 antennas

The XXX_SWITCH_ANTENNA1/2 actions keep track of switching between
3 single antennas or between 3 pairs in case of MIMO2 on a MIMO3 device.
As current and future chips will have at most 2 antennas drop these.
While at it also convert the actions into enums and cleanup
the code a bit.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>