Bluetooth: Kconfig: Merge BT_HCI_VS_EXT into BT_HCI_VS

The naming of these two options was problematic, since it's both of them
are about vendor extensions, even though one has _EXT in the name and
the other doesn't. Just merge one option into the other. This has a
slight overhead on the controller side of enabling some more vendor
features if BT_HCI_VS is enabled, but that should hopefully be
acceptable.

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