ASoC: cs35l35: Correctly handle 0 for bst_ipk

Zero is a totally valid value to specify for the bst_ipk, as such we
should append CS35L35_VALID_PDATA to ensure that it actually makes it
into the register value.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>