soc: qcom: Validate read and write index before calculating ptr

Currently we are not validating read and write index of
tx and rx fifo's before calculating ptr, this can lead to
out-of-bound access. The patch adds proper check for the same.

Change-Id: I7b158e94ae743a90ac364783fe31914ca0fa582b
Signed-off-by: Hardik Arya <harya@codeaurora.org>