scsi: esas2r: use ktime_get_real_seconds()

do_gettimeofday() is deprecated because of the y2038 overflow.  Here, we
use the result to pass into a 32-bit field in the firmware, which still
risks an overflow, but if the firmware is written to expect unsigned
values, it can at least last until y2106, and there is not much we can
do about it.

This changes do_gettimeofday() to ktime_get_real_seconds(), which at
least simplifies the code a bit, and avoids the deprecated
interface. I'm adding a comment about the overflow to document what
happens.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>