sfc: don't call dma_supported

dma_set_mask already checks for a supported DMA mask before updating it,
the call to dma_supported is redundant.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Solarflare linux maintainers <linux-net-drivers@solarflare.com>
Cc: Shradha Shah <sshah@solarflare.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>