Bluetooth: Refactor bt_recv() logic

Refactor the bt_recv() logic to make it easier to insert #ifdefs to
remove the RX queue and thread when the context calling bt_recv()
itself is sufficient enough.

Change-Id: Ie8c5a4dfe9533a4464a2e4909c94d3e1b185b55b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>