[SCSI] fix oops in all legacy host adapters caused by 6f381fa3

Commit 6f381fa3
Author: Lin Ming <ming.m.lin@intel.com>

[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 <viro@zeniv.linux.org.uk>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>