platform/x86: fujitsu-laptop: Don't oops when FUJ02E3 is not presnt

My Fujitsu-Siemens Lifebook S6120 doesn't have the FUJ02E3 device,
but it does have FUJ02B1. That means we do register the backlight
device (and it even seems to work), but the code will oops as soon
as we try to set the backlight brightness because it's trying to
call call_fext_func() with a NULL device. Let's just skip those
function calls when the FUJ02E3 device is not present.

Cc: Jonathan Woithe <jwoithe@just42.net>
Cc: Andy Shevchenko <andy@infradead.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: <stable@vger.kernel.org> # 4.13.x
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>