USB: serial: clean up chars_in_buffer

No need to grab disconnect mutex in chars_in_buffer now that no
sub-driver is or should be querying hardware buffers anymore. (They
should use wait_until_sent.)

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