ANDROID: serdev: restrict claim of platform devices

Make the fallback path for claiming platform devices trigger only if a
new module parameter is specified:

  serdev_ttyport.pdev_tty_port=ttyS2

Bug: 146517987
Change-Id: Ibf331ad6e6d8712a405921530f217f7122428b13
Signed-off-by: Alistair Delva <adelva@google.com>