mmc: fix capacity calculation when EXT_CSD_SEC_CNT is used

Since the type of "ext_csd" was array of char, the following
calculation might fail when the value of ext_csd[EXT_CSD_SEC_CNT]
was minus.

	capacity = ext_csd[EXT_CSD_SEC_CNT] << 0
			| ext_csd[EXT_CSD_SEC_CNT + 1] << 8
			| ext_csd[EXT_CSD_SEC_CNT + 2] << 16
			| ext_csd[EXT_CSD_SEC_CNT + 3] << 24;

So, this patch changes the type of "ext_csd" to array of u8.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>