Bluetooth: Controller: Use bt_buf_get_rx for buffer allocation

The controller uses K_FOREVER for the allocations so there's no
benefit in trying to use specialized pools (which exist to try to
guarantee availability of buffers for critical data).

Change-Id: I130f2c44a2f28af1284290e6a0b17dcba438862f
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>