Bluetooth: Fix potential race condition in bt_pub_key_gen()

It's possible for the Public Key to be available by the time that the
bt_hci_cmd_send_sync() returns, so we need to make sure the flags have
the right values no matter what.

Change-Id: I053093b6611af360f52b14ddca50d409388f9475
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>