asoc: wcd937x: disable TX CH2 after power down

TX channel2 is not disabled after power down. It leads to
noise at the beginning of next recording on CH2.
Disable it after power down.

Change-Id: If12a84df52b9c8cd55de2bdcb91f950b054898b8
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>