Fix stack buffer size for IPC writev messages

The stack buffer for IPC messages was 16 bytes, limiting messages to a
size of 4 (each message is 32 bit).
However, the touch screen driver is trying to send messages of size 5....

(AC: Set to 20 bytes having checked the max size allowed)

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>