mmc: Change 'part_config' to be a u8 not char.

In some places we check if part_config is set to MMCPART_NOAVAILABLE
(0xff).  With part_config being a char this is always false.  We should
be using a u8 to store this value instead, after a quick consultation
with the Linux Kernel.  Reported by clang-3.8.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@konsulko.com>

include/mmc.h

@@ -430,7 +430,7 @@ struct mmc {
 	u8 part_support;
 	u8 part_attr;
 	u8 wr_rel_set;
-	char part_config;
+	u8 part_config;
 	uint tran_speed;
 	uint read_bl_len;
 	uint write_bl_len;