ASoC: Fix wm8753 register cache size and initialization

Register cache space was not being allocated for the final register,
causing bugs when it was used.  Allocate space for it.

Also ensure that the final register is displayed in sysfs.

[Commit message rewritten to document actual issue. -- broonie]

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>