nl/cfg80211: advertise OBSS scan requirement

wpa_supplicant will do OBSS scan for drivers that implement
auth/assoc API. Drivers that implement nl80211 connect API
(rather than auth/assoc) may need wpa_supplicant to do this
as well.

Add a new feature flag to inform it (wpa_s) that a driver
needs wpa_supplicant to do OBSS scans.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>