scsi: myrb: Don't use GFP_DMA in myrb_pdev_slave_alloc()

The driver doesn't express DMA addressing limitation under 32-bits anywhere
else, so remove the spurious GFP_DMA allocation.

Link: https://lore.kernel.org/r/20211222091801.924745-1-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>