fbdev: xilinxfb: Make xilinxfb_release() return void

The function xilinxfb_release() returns zero unconditionally. Make it
return void. There is no semantic change, the only effect is that it
becomes obvious that the driver's .remove() callback always returns zero.

This is a preparation for making platform remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>