RDMA/siw: Fix pointer-to-int-cast warning in siw_rx_pbl()

The variable buf_addr is type dma_addr_t, which may not be the same size
as a pointer.  To ensure it is the correct size, cast to a uintptr_t.

Fixes: c536277e

 ("RDMA/siw: Fix 64/32bit pointer inconsistency")
Link: https://lore.kernel.org/r/20200610174717.15932-1-tseewald@gmail.com
Signed-off-by: Tom Seewald <tseewald@gmail.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>