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>