net: openthread: Fix utilsFlashErasePage function
net: openthread: Fix utilsFlashErasePage function

Zephyr implementation of OpenThreads utilsFlashErasePage platform
function did not disable flash protection before calling `flash_erase`
function. This resulted in an error instead of actual flash erase on
platforms that properly implement flash write protection.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

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