Reduce CPU utilization from ~60% to ~10% on busy HCI reads.

This patch improves RFCOMM throughput and reduces CPU utilization.
Instead of using a counting semaphore to measure bytes and reading
one at a time from the eager reader's buffer, read in bulk based
on the incoming read request size.

Change-Id: I17046bfbc3ca49576a9c82b38911aeb84234881a