USB: serial: fix infinite wait_until_sent timeout

Make sure to handle an infinite timeout (0).

Note that wait_until_sent is currently never called with a 0-timeout
argument due to a bug in tty_wait_until_sent.

Fixes: dcf01050 ("USB: serial: add generic wait_until_sent
implementation")
Cc: stable <stable@vger.kernel.org>	# v3.10

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>