driver: usb: nordic: Fix compile warning with newlib
driver: usb: nordic: Fix compile warning with newlib

This commit fixes compile warning with CONFIG_NRFX_USB
and CONFIG_NEWLIB_LIBC.

Signed-off-by: Takumi Ando <0478d715f8a62f7857cbc5e2b027c3732461c480@atmark-techno.com>

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