OMAP: DSS2: OMAPFB: Fix sysfs mirror input check

Using bool silently converted input to 0 or 1, making the range check
useless. Use unsigned long instead, and convert to bool later. Found by
Coverity.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>