sound: ua101: fix division by zero at probe

In the Linux kernel, the following vulnerability has been resolved:

sound: ua101: fix division by zero at probe

Add a missing sanity check for bNrChannels in detect_usb_format()

to prevent a division by zero in playback_urb_complete() and

capture_urb_complete().

USB core does not validate class-specific descriptor fields such

as bNrChannels, so drivers must verify them before use. If a

device provides bNrChannels = 0, frame_bytes becomes zero and is

later used as a divisor in the URB completion handlers, leading

to a kernel crash.