Bluetooth: Fix public key callback management

Clear the callback list once generation is complete and we've done
calling all callbacks. This lets us use bt_pub_key_gen() multiple
times, which before this patch could have resulted in a corrupt linked
list.

Also remove redundant callback dispatching from bt_pub_key_gen() since
the function checks for the PUB_KEY_BUSY flag in the beginning, i.e.
there cannot be other pending generation actions at this point in the
code.

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