msm_serial_hs: Fix the receive buffer index pointer in logging

The receive buffer should point to the offset based on the
BAM descriptor being used. Update accordingly so that correct
snapshot of the buffer is logged.

Change-Id: Ife9fa6596a2d476ed7d39e7551b61e9c7cacd625
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>