cfg80211: track time using boottime

The cfg80211 layer uses get_seconds() to read the current time
in its supend handling. This function is deprecated because of the 32-bit
time_t overflow, and it can cause unexpected behavior when the time
changes due to settimeofday() calls or leap second updates.

In many cases, we want to use monotonic time instead, however cfg80211
explicitly tracks the time spent in suspend, so this changes the
driver over to use ktime_get_boottime_seconds(), which is slightly
slower, but not used in a fastpath here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>