Bluetooth: Decrease Rx processing priority

The following error and warning is received when connecting
to certain central devices:
[bt] [ERR] hci_num_completed_packets: packets count mismatch
[bt] [WRN] bt_att_recv: Ignoring unexpected request

This could happen if Tx-ed packet is not added to pending
list before a num of completed packets event and/or new Rx
packet is received.

This is fixed by reducing the Rx thread priorities in the
Host and the Controller.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>