nl80211: check NL80211_ATTR_SCHED_SCAN_INTERVAL only once

The presence of the NL80211_ATTR_SCHED_SCAN_INTERVAL attribute was
checked in nl80211_parse_sched_scan() and
nl80211_parse_sched_scan_plans() which might be a bit redundant
so removing one.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>