USB: serial: f81534: fix division by zero on line-speed change

commit 188c9c2e upstream.

The driver leaves the line speed unchanged in case a requested speed is
not supported. Make sure to handle the case where the current speed is
B0 (hangup) without dividing by zero when determining the clock source.

Fixes: 3aacac02 ("USB: serial: f81534: add high baud rate support")
Cc: stable@vger.kernel.org      # 4.16
Cc: Ji-Ze Hong (Peter Hong) <hpeter@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>