ASoC: Fix masking of WM8962 FLL1 register

When configuring the FLL we preserve the FLL enable configuration in order
to allow us to reenable the FLL after configuration but we do not clear
the other bits in the register, causing old configuration to be preserved.

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