ath10k: use 64-bit crash dump timestamps

The dump format uses 64-bit timestamps already, but calling
getnstimeofday() only returns a 32-bit number on 32-bit architectures,
so that will overflow in y2038.

This changes it to use ktime_get_real_ts64() instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>