ASoC: Correct code taking the size of a pointer

sizeof(codec->reg_cache) is just the size of the pointer.  Elsewhere in the
file, codec->reg_cache is used with sizeof(wm8900_reg_defaults), so the
code is changed to do the same here.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

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

*f(...,(T)x,...)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>