dfu:ext4:fix Fix ext4{read|write} command formatting

dfu:ext4:fix Fix ext4{read|write} command formatting

In the following commit:
"dfu: Support larger than memory transfers."
SHA1: ea2453d56b8860dbd18a3c517531ffc8dcb5c839

The ext4{read|write} command formatting has been changed. It removed
a write mandatory [sizebytes] parameter.

It extents DFU_FS_EXT4 case at mmc_file_op to provide mandatory
parameter for DFU write.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Tom Rini <trini@ti.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>