iwlwifi: mvm: Don't call iwl_mvm_sta_from_mac80211() with NULL sta

iwlwifi: mvm: Don't call iwl_mvm_sta_from_mac80211() with NULL sta

The recent fix for NULL sta in iwl_mvm_get_tx_rate() still has a call
of iwl_mvm_sta_from_mac80211() that may be called with NULL sta.
Although this practically only points to the address and the actual
access doesn't happen due to the conditional evaluation at a later
point, it looks a bit flaky.

This patch drops the temporary variable above and evaluates
iwm_mvm_sta_from_mac80211() directly for avoiding confusions.

Fixes: d599f714b73e ("iwlwifi: mvm: don't crash on invalid rate w/o STA")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20220121114024.10454-1-tiwai@suse.de
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>