usb: netusb: Replace IS_ENABLED(VERBOSE_DEBUG) with just VERBOSE_DEBUG

IS_ENABLED() is only useful for macros that might be undefined.
VERBOSE_DEBUG is defined at the top of these files.

Also makes it harder to confuse for for Kconfig references.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>