dfu: allow read with no data without error for EOF indication

dfu: allow read with no data without error for EOF indication

This patch allows the DFU backend to indicate that that it can't
provide no more data to fill the DFU buffer, by setting b_left =0
without error, even if the size of received data is lower of the
expected total size indicated by get_medium_size.

For USB DFU stack point of view, it is acceptable:
the read length < requested size in DFU_UPLOAD and the
transaction is stopped.

That avoid infinite loop issue in dfu_read_buffer_fill because the
size for the DFU read is limited by get_medium_size = r_left
and the DFU stack expects that read is allowed up to this size.

This issue never occurs for current flash device (where chunk are
always completely read, and b_left will be never 0) but it is useful for
virtual partition when the backend only know the max size of this
alternate, the real size of the data are only known in the read
treatment.

PS: for file access on mmc, EOF is never reached as
    dfu_get_medium_size_mmc returns the exact size of the file.

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