drm/i2c: tda998x: fix bad value in the AIF

The AIF has an uninitialized byte. This patch clears the whole buffer
before filling it.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>