mmc: cull sg list to match mmc request size

mmc layer may introduce additional (compared to block layer) limits on
request size. Culling of the sg list to match adjusted request size
simplifies the handling of such cases in the low level driver, allowing
it to skip block count checks while processing sg entries.

(fixes for wbsd and sdhci by Pierre Ossman)

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>