scsi: smartpqi: limit transfer length to 1MB

The smartpqi firmware will bypass the cache for any request larger than
1MB, so we should cap the request size to avoid any performance
degradation in kernels later than v4.3

This degradation is caused from d2be537c,
which changed max_sectors_kb to 1280k, but the hardware is able to
work fine with it, so the true fix should be from smartpqi driver.

Signed-off-by: Yadan Fan <ydfan@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Acked-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>