leds: ledtrig-activity: use ktime_get_boot_ns()

get_monotonic_boottime() is deprecated, so let's convert this to
the simpler ktime_get_boot_ns().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>