scsi: mpt3sas: Don't change the DMA coherent mask after allocations

The DMA layer does not allow changing the DMA coherent mask after there are
outstanding allocations.

Link: https://lore.kernel.org/r/1587626596-1044-2-git-send-email-suganath-prabu.subramani@broadcom.com
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Suganath Prabu <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>