dfu: add missing dependency for SPI flash DFU driver

dfu: add missing dependency for SPI flash DFU driver

Building the SPI flash DFU driver fails if SPI flash support is missing.

    drivers/dfu/dfu_sf.c:123:29: error:
    ‘CONFIG_SF_DEFAULT_MODE’ undeclared (first use in this function);

Add the missing dependency.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20240604054425.105902-1-heinrich.schuchardt@canonical.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>