drivers/dma: drop unnecesary memset

memset of 0 is not needed after kzalloc

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression x;
statement S;
@@

x = kzalloc(...);
if (x == NULL) S
... when != x
-memset(x,0,...);// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>