fbdev: amifb: Mark driver struct with __refdata to prevent section mismatch warning

As described in the added code comment, a reference to .exit.text is ok
for drivers registered via module_platform_driver_probe(). Make this
explicit to prevent a section mismatch warning.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Helge Deller <deller@gmx.de>