thunderbolt: Correct the size argument to devm_kzalloc

thunderbolt: Correct the size argument to devm_kzalloc

nhi->rx_rings does not have type as struct tb_ring *, as it is a
double pointer so the elements of the array should have pointer type,
not structure type.

The Coccinelle semantic patch that makes this 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>
Cc: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>