[SCSI] fix oops in all legacy host adapters caused by 6f381fa

[SCSI] fix oops in all legacy host adapters caused by 6f381fa

Commit 6f381fa344911d5a234b13574433cf23036f9467
Author: Lin Ming <[email protected]>

[SCSI] scsi_lib: use correct DMA device in __scsi_alloc_queue

Caused a regression where we oops in every legacy mode SCSI host driver
because they supply a NULL pointer to scsi_add_host().  Fix this by checking
for the NULL in scsi_add_host_with_dma() and changing the DMA device to being
the platform_bus in that case (which replicates the original behaviour).

Reported-by: Al Viro <[email protected]>
Signed-off-by: James Bottomley <[email protected]>