Bluetooth: Use bt_hci_cmd_send_sync in set_ad to catch errors

Using send_sync lets us return an immediate error to the application
if the HCI command fails. This also reduces the pressure on available
buffers since we don't allocate multiple command buffers before
starting to process the command queue.

Change-Id: I5613e4e9dd8dcc0db45bad051d7c4980b49e428d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>