bthelper: FIX: Only run for onboard BT modems

The test designed to restrict the changes to onboard BT modems (those
attached by a UART) was correct, but the action taken on a failure
didn't end the script as it should have (the "exit 0" was run in a
sub-shell, which only ended the sub-shell).

Fix that.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>