soc: qcom: Update conditional logic for validating read and write index

Currently conditional logic is not proper for validating read and write
index of tx and rx fifo's before calculating ptr. The patch updates
the same.

Change-Id: I144db249eb47cc922bf0734a367a949b37eddef7
Signed-off-by: Hardik Arya <harya@codeaurora.org>