usb: misc: ks_bridge: Fail probe if port already in use

If a ksb port with a particular device ID is in use, and the user
connects another device with the same device ID, then we end up
overriding the ksb members with the new values leading to
functionality failure of the first device, among other possible
issues. To avoid that, fail the probe.
While at it, also fix minor checkpatch warning.

Change-Id: I2a5901b8fe49b69b95e3a842c4efcd976425c106
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>