ASoC: Fix off-by-one bug in WM8962 register cache size configuration

This is a simple off-by-one bug, the size of the register cache is
incorrectly set to the maximum register index. Fix it by adding one.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>