cxl: drop the dma_set_mask callback from vphb

The CXL code never even looks at the dma mask, so there is no good
reason for this sanity check.  Remove it because it gets in the way
of the dma ops refactoring.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>