usb: class: dfu: fix firmware upload
usb: class: dfu: fix firmware upload

The flash_read() function takes an offset from beginning of the flash.
This patch subtract FLASH_BASE_ADDRESS from the absolute address. This
fixes DFU firmware upload.

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

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