Bluetooth: drivers/nble: Require callback for bt_enable()

The nble nRF51 firmware cannot handle situations where it receives
data before it is ready. The RPC calls otoh cannot be blocking.
Combine this with the fact that bt_enable(NULL) is supposed to be
blocking means we cannot support this mode of operation with nble.

Change-Id: Ib9c2c322b44b04bc48be243c2ba1c1bc4becb8ea
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>