fbdev: lxfb: Fix potential divide by zero

var->pixclock can be assigned to zero by user. Without proper
check, divide by zero would occur in lx_set_clock.

Error out if var->pixclock is zero.

Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>