Bluetooth: drivers: Don't use deprecated NANOKERNEL init level

Don't use the deprecated NANOKERNEL init level. The only requirement
for Bluetooth drivers is for them to be registered before the
application main() runs, so POST_KERNEL should be good enough.

Change-Id: I02a8609bf63e9d608b802576214a2e76211b3965
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>