sfc: fix build warnings on 32-bit

Truncation of DMA_BIT_MASK to 32-bit dma_addr_t is semantically safe,
 but the compiler was warning because it was happening implicitly.
Insert explicit casts to suppress the warnings.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>