cdc_acm: fix interrupt handling, API registration, and propgate errors.

As per uart_pipe.c, irq_is_pending should only return true if any
*unmasked* interrupts are pending.

The device_get_binding() code only checks devices that have an API.
Set the API at compile time so that other devices can refer to the CDC
ACM driver.

usb_write() may return EBUSY if the endpoint is full.  Propagate this
for future use.

Signed-off-by: Michael Hope <mlhx@google.com>