iwlwifi: mvm: remove check for vif in iwl_mvm_vif_from_mac80211()

This check is useless, because we would return NULL in that case and
none of the callers actually check that the return value was not NULL
before accessing it.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210802172232.f1c4844a2650.If4dc009e22cda51099a1dc4237d04bf4313055d6@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>