Bluetooth: nble: Move receive logic to fiber

Move receiving logic to fiber rx_fiber(). rx_queue is registered with
uart driver and all data is sent there for processing. Clean up old
ipc_channel interface since there will be only one interface to uart
driver.

Change-Id: I292863b1f38e5adb8ca4e6ac63aed09c83de56bf
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>