bam_dmux: Adapt RX timer interval to data rate.

Instead of always sleeping for 950 to 1050
microseconds between subsequent reads of the
BAM RX pipe, allow the driver to pick an
interval based on how full the BAM pipe is.
Using this technique allows the CPU to stay
in an idle state longer than with just 1ms
sleeps without compromising performance.

Change-Id: I996cd3a9da912a0acc5552566a093d3904cec4e7
Signed-off-by: Anurag Singh <anursing@codeaurora.org>