usb: Simplify the parameter parsing of the legacy usb serial device

Coverity complains about the current code, so let's get rid of
the now unneeded while loop and simply always emit "unrecognized
serial USB option" for all unsupported options.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1495177204-16808-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
