mwifiex: advertise low priority scan feature

Low priority scan handling code which delays or aborts scan
operation based on Tx traffic is removed recently. The reason
is firmware already takes care of it in our new feature scan
channel gap. Hence we should advertise low priority scan
support to cfg80211.

This patch fixes a problem in which OBSS scan request from
wpa_supplicant was being rejected by cfg80211.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>