Bluetooth: h4: Use k_fifo instead of k_sem

So far the use of k_sem meant that there was no major benefit of
having more than 2 or 3 RX buffers since there was no queuing
mechanism. Instead of using k_sem, introduce a k_fifo and use that to
queue up incoming buffers. This way the RX buffer count can be
increased with measurable effects on throughput.

Change-Id: I8122b233aeee7c8e145de3fff5f10bcfe348efaa
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>