usb: cdc_acm: Update log level only if UART log backend is enabled

Change updates log level only if UART log backend is enabled in Kconfig
configuration. The log level update is not needed in case logs are
provided over other backend (e.g. RTT).

Change affects both USB stacks.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>