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>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 0360d9d..9712047 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -68,6 +68,7 @@
 
 config DFU_SF
 	bool "SPI flash back end for DFU"
+	depends on SPI_FLASH || DM_SPI_FLASH
 	help
 	  This option enables using DFU to read and write to SPI flash based
 	  storage.