dfu:ext4:fix Fix DFU upload functionality

For the first eMMC read of data for upload, use the "large" dfu_buf (now
configurable) instead of usb request buffer allocated at composite layer
(which is 4KiB) [*].

For eMMC the whole file is read, which usually is larger than the buffer [*]
provided with usb request.

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>