MLK-10001: ASoC: fsl_sai: no sound for mono wav in master mode

The bclk caculation should according to the slot num, not the channels.
Because sometime we have two slots, but only one slot is enabled for mono
channel.
As when the codec wm8962 works on mono mode, it needs two slots I2S signal.
So here set the default slots of sai to 2, and add function set_tdm_slots for
future usage.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
(cherry picked from commit 991a1f269ce4e4d0daa2cf5615169891acca0607)