qtnfmac: remove set but not used variable 'vif'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/quantenna/qtnfmac/cfg80211.c: In function 'qtnf_dump_survey':
drivers/net/wireless/quantenna/qtnfmac/cfg80211.c:694:19: warning:
 variable 'vif' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>