udlfb: Fix invalid return codes in edid sysfs entry store function

Return a negative errno instead of zero in the write function of
the sysfs entry in case of error.
Also add a check on the return value of dlfb_setup_modes().

Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Acked-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>