cfg80211/nl80211: add interval attribute for scheduled scans

Introduce NL80211_ATTR_SCHED_SCAN_INTERVAL as a required attribute for
NL80211_CMD_START_SCHED_SCAN.  This value informs the driver at which
intervals the scheduled scan cycles should be executed.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>