[SCSI] ibmvscsi: Fixup desired DMA value for shared memory partitions

When running ibmvscsi in a shared memory partition, it must provide
a default value for the amount of DMA resources it will need in order to
perform reasonably well. This was being calculated in sectors rather than
bytes, as it should. This patch fixes this.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>