rtc-at91sam9: Correct size given to memset

Memset should be given the size of the structure, not the size of the
pointer.

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

)

// <smpl>
@@
type T;
T *x;
expression E;
@@

memset(x, E, sizeof(
+ *
 x))
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>