ASoC: cs35l35: Correct handling of PDN_DONE with external boost

When using an external boost supply the PDN_DONE bit is not set, update
the handling in this case to use to use an appropriate fixed delay.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>