ASoC: msm: use separate variable for Tx bit rate

Using a separate variable to track bit rate for Tx
use cases, will avoid any issue while having Rx and
Tx sessions with different bit rates.

CRs-Fixed: 777328
Change-Id: I26826795b54a49a56d77087aa69cc707325018c1
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>