Bluetooth: Host: Allow setting more than 251 bytes of adv data

When the application sets more than 251 bytes of advertising data,
the data is sent over multiple HCI commands.
This is only allowed if the advertiser is not running
as defined by the HCI specification.

The data is sent to the controller one AD-field at the time.
If an AD-field is larger than BT_HCI_LE_EXT_ADV_FRAG_MAX_LEN,
the data is split over two commands.
This introduces some additional complexity.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>