dfu: allow backend to specify a maximum buffer size

CONFIG_SYS_DFU_DATA_BUF_SIZE may be large to allow for FAT/ext layouts
to transfer large files. However, this means that individual write
operations will take a long time. Allow backends to specify a maximum
buffer size, so that each write operation is limited to a smaller data
block. This prevents the DFU protocol from timing out when e.g. writing
to SPI flash. I would guess that NAND might benefit from setting this
value too, but I can't test that.

Signed-off-by: Stephen Warren <swarren@nvidia.com>