iwlwifi: mvm: remove pointless NULL assignment

Assigning mvmsta to be NULL when we are about to exit the
function is pointless. Remove it. Move the variable declaration
to the scope it is used.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>