drivers: udc_rpi_pico: remove confusing error log

With the commit c3109b9ebe67
("usb: device_next: cdc_acm: rx throughput improvements"),
the CDC implementation queues as many buffers as are available. This may
trigger an error log, "<err> udc_rpi_pico: Endpoint 0x01 busy".
There is nothing wrong with the driver, it is just that another transfer
is queued while an endpoint is busy.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>