usb-serial: Move USB_TOKEN_IN into a helper function

We'll be adding a loop, so move the code into a helper function.  breaks
are replaced with returns.  While making this change, add braces to
single line if statements to comply with coding style and keep
checkpatch happy.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Message-id: 20200316174610.115820-2-jandryuk@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>