usb: class: dfu: fix support for SPI based flash
usb: class: dfu: fix support for SPI based flash

With an SPI based flash, CONFIG_FLASH_BASE_ADDRESS is not defined. In
that case it is safe to assume that the base address is 0.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr