wlcore: use boottime for fw time sync

Using getnstimeofday()/timespec_to_ns() causes an overflow on 32-bit
architectures in 2038, and may suffer from time jumps due to
settimeofday() or leap seconds.

I don't see a reason why this needs to be UTC, so either monotonic
or boot time would be better here. Assuming that the fw time keeps
running during suspend, boottime is better than monotonic, and
ktime_get_boot_ns() will also save the additional conversion to
nanoseconds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>