Add the attribute QCA_WLAN_VENDOR_ATTR_SCAN_PRIORITY to configure the
priority of vendor scan relative to other scan requests.
Add the valid values that this attribute can take.
Also, add an API to convert NL80211-based scan priority value to internal
scan priority value.
Change-Id: I6c62166c2dd6ea0880b332496b1e4bb0b41840c6
CRs-Fixed: 3044332