ASoC: wm8940: Fix mask for setting BCLKDIV

According to the datasheet:
BCLK is controlled by BIT[4:2] of WM8940_CLOCK(06h) register.

Signed-off-by: Chris Paulson-Ellis <chris@edesix.com>
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>