iwlwifi: fix double assign in iwl_start_tx_ba_trans_ready()

"vif" is assigned twice.  We can remove the first one.

This silences a Smatch warning that "ctx" could be one step past the
end of the priv->contexts[] array.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>