USB: serial: f81232: fix division by zero on line-speed change

USB: serial: f81232: fix division by zero on line-speed change

commit 7f226a574f446a5567683f12e9eb7b175cf20fa5 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: f4dc6d6d0e19 ("USB: serial: f81232: add high baud rate support")
Cc: stable@vger.kernel.org      # 5.2
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>