drivers: usb: nordic: Do not log an error if HFCLK is busy

HFCLK may be requested for multiple modules, i.e. Bluetooth stack.
When any other module has requested HFCLK to run, the driver will
return -EBUSY on free attempt which is not an error - thie means
that free request has been processed, but someone else is still
requiring the clock to run. When all clock users free the clock,
it may be disabled.

Related issue: #15145

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>