dfu: dfu_sf: Read the SPI flash in 16 MiB chunks

Not all SPI flashes and controllers can do continuous transfer longer
than 16 MiB, so perform the DFU read in 16 MiB chunks.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>