Bluetooth: Refactor nRF51 API

The previous enable/disable API implementations weren't actually what
their names implied, but simply gave the right result for the
initialization scenario when called after each other. Split these into
proper init/enable/disable APIs which do the exact thing that the
names imply.

Change-Id: I09a930d3607f4919ecd889ec3ee4ba8d7b12ee36
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>