ASoC: Mark WM5100 register map cache only when going into BIAS_OFF

commit e53e4173 upstream.

Writing to the registers won't work if we do actually manage to hit a fully
powered off state.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>