arm: imx: Zero entire imxdma structure

The semantic match that finds the problem:
// <smpl>
@@
type T;
identifier x;
@@

T *x;
...
* memset(x, ..., ... * sizeof(x) * ...);
// </smpl>

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>