fix(mmc): align part config type

This corrects MISRA C2012 10.3: The value of an expression shall not be
assigned to an object with a narrower essential type or of a different
essential type category.
Set part_config to be an unsigned char everywhere it is used.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I9cf87848fb016a5630b3c8091290419e3bc62b91