ASoC: WM8731: Fix incorrect mask for bypass path disable

According to the datasheet the bypass path enable/disable is
bit 3 therefore we need 0x8 and not 0x4.

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>