Bluetooth: HCI: Use lower-case values for bus and quirks in devicetree

It's more common (and more readable) convention to use lower-case
names for string-based device tree property values. Convert the HCI bus
and quirks to follow this convention. Also take advantage of the
recently added support for string-array enums to enforce that the
correct values are used.

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