dfu: allow dfu read on partition greater than 2GB

dfu: allow dfu read on partition greater than 2GB

solve issue on get_medium_size() function
the detection of error is a simple test < 0
but for ARM platform, long is 32bits and 2GB = 0x80000000
is seen as error.

I solve the issue by changing the prototype fo the function
to separate size and result.
This patch prepare the next patch with size change to u64.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>