USB: sierra: fix characters being dropped at close

Fix characters potentially being dropped at close due to missing
chars_in_buffer implementation.

Note that currently the write urbs are not even killed at close (will be
fixed separately), but this could still lead to dropped data since we
have lowered DTR/RTS.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>