ASoC: wm8994: Fix setting rate_reg for wm8994-aif2

For wm8994-aif2, the rate_reg should be WM8994_AIF2_RATE.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org