Input: synaptics-rmi4 - fix video buffer size

The video buffer used by the queue is a vb2_v4l2_buffer, not a plain
vb2_buffer. Using the wrong type causes the allocation of the buffer
storage to be too small, causing a out of bounds write when
__init_vb2_v4l2_buffer initializes the buffer.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Fixes: 3a762dbd

 ("[media] Input: synaptics-rmi4 - add support for F54 diagnostics")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20191104114454.10500-1-l.stach@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>