input: touchscreen: mainstone: fix pxa2xx+pxa3xx configuration

There are two different ways of flushing the ac97 queue
in this driver, selected by a compile time option.

Change this to a runtime selection to make it work when both
are enabled.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: linux-input@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>