Bluetooth: Call bt_recv from priority higher that TX thread.

A build assert in dummy.c lists the following requirement:
[...] receive thread priority shall be higher than the Bluetooth
Host's Tx and the Controller's receive thread priority.
This is required in order to dispatch Number of Completed Packets
event before any new data arrives on a connection to the Host threads.

The drivers uses a priority that is equal to the Host TX thread,
and since they don't use the CONFIG define that is only available
to the controller then this BUILD_ASSERT will not catch the
requirement.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>