scsi: myrs: fix build failure on 32 bit

For 32 bit versions we have to be careful about divisions of 64 bit
quantities so use do_div() instead of a direct division.  This fixes a
warning about _uldivmod being undefined in certain configurations

Fixes: 77266186

 ("scsi: myrs: Add Mylex RAID controller")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>