mvmvif is defined and set in rs_mimo_allow but not used. Compiling
iwlwifi with W=1 gives the following warning, remove it. mvmsta is
used only to obtain mvmvif so remove it as well.

iwlwifi/mvm/rs.c: In function 'rs_mimo_allow':
iwlwifi/mvm/rs.c:165:22: warning: variable 'mvmvif' set but not used.[-Wunused-but-set-variable]

This fix removes calls to iwl_mvm_sta_from_mac &
iwl_mvm_vif_from_mac. They are both accessors, and do not have any
side-effects.  Commit e621c228 ("iwlwifi: rs: Remove workaround
that disables MIMO on P2P") removed a workaround that disabled MIMO on
P2P, 'mvmvif' was used for that workaround, but not removed with it.

Fixes: e621c228 ("iwlwifi: rs: Remove workaround that disables MIMO on P2P")
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Cc: Alexander Bondar <alexander.bondar@intel.com>
Cc: Emmanuel Grumbach <emmmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>