dfu: mmc: rearrange the code

dfu: mmc: rearrange the code

Rename functions for bufferred file io operations to make them easier to
understand. Also add missing file offset argument to them (currently
unused). All this is a preparation to remove predefined file size limit
(CONFIG_SYS_DFU_MAX_FILE_SIZE) for DFU read/write operations.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>