mmc: fix spares errors of sdhci.c

1. sdhci_prepare_data: fix shadowing of count variable
	u8 count
	int count -> sg_cnt;
2.  sdhci_add_host: assignment of integer to pointer
	dma_mask = 0 -> dma_mask = NULL;

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>