mtd: physmap_of: Correct the size argument to kzalloc

mtd_list has type struct mtd_info **, not struct mtd_info *, so the
elements of the array should have pointer type, not structure type.

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

)

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

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

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>