iwlwifi: mvm: refactor iwl_mvm_sta_rx_agg()

Refactor the firmware call in iwl_mvm_sta_rx_agg() out into
its own function to be able to change it more easily in the
next patch.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220128153014.8ec17e282b24.I37c008a9141a2868ee4560e6de303e8bfbb93502@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>