scsi: hpsa: limit transfer length to 1MB, not 512kB

e2c7b433 was supposed to limit transfer length to 1MB, but got the unit of
max_sectors wrong.

Fixes: e2c7b433 ("scsi: hpsa: limit transfer length to 1MB")
Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>