nl80211: Add option to delay start of schedule scan plans

nl80211: Add option to delay start of schedule scan plans

The userspace may want to delay the the first scheduled scan.
This enhances sched_scan to add initial delay (in seconds) before
starting first scan cycle. The driver may optionally choose to
ignore this parameter and start immediately (or at any other time).

This uses NL80211_ATTR_SCHED_SCAN_DELAY to add this via user
global configurable option: sched_scan_start_delay.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>