auxdisplay: Call charlcd_backlight in place

This moves the call to charlcd_backlight from the end of the switch
into the actual case statement that originates the change of the
backlight. This is more consistent to what is now found in this switch.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>