mwifiex: use ktime_get_real for timestamping

The mwifiex_11n_aggregate_pkt() function creates a ktime_t from
a timeval returned by do_gettimeofday, which is slow and causes
an overflow in 2038 on 32-bit architectures.

This solves both problems by using the appropriate ktime_get_real()
function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Amitkumar Karwar <akarwar@marvell.com>
Cc: Nishant Sarmukadam <nishants@marvell.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>