ASoC: wm8741: Allow master clock switching

The set of supported sample rates depends on the master clock supplied
to the codec. Allow the machine driver to set the required master clock
in hw_params().

Signed-off-by: Sergej Sawazki <ce3a@gmx.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>