scsi: megaraid: use ktime_get_real for firmware time

do_gettimeofday() overflows in 2038 on 32-bit architectures and is
deprecated, so convert this driver to call ktime_get_real()
directly. This also simplifies the calculation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>