powerpc/mpic_msgr: Use kcalloc and correct the argument to sizeof

mpic_msgrs has type struct mpic_msgr **, not struct mpic_msgr *, so the
elements of the array should have pointer type, not structure type.
The advantage of kcalloc is, that will prevent integer overflows which
could result from the multiplication of number of elements and size and
it is also a bit nicer to read.

The Coccinelle semantic patch that makes the first change is as follows:

// <smpl>
@disable sizeof_type_expr@
type T;
T **x;
@@

  x =
  <+...sizeof(
- T
+ *x
  )...+>
// </smpl>

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>