drivers: usb: Start Nordic driver workqueue at USB init.

This patch makes the initialization of workqueue earlier
for Nordic driver. Without this change Nordic devices
will fail tests/subsys/usb/device test as it is resetting
USB and by that will try to start already started workqueue
thread. This may lead to Zephyr fatal error.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>