tripplite_usb: silence warning (0.23)

Pedantic, to be sure, but someone might try the driver with a protocol not
listed, and sure enough, bv_12V won't be initialized.
tripplite_usb: last tweaks, for now.

Initialize bv_12V to a dummy value, since gcc can't see that it is used in the
union of both conditionals where it is set.

Also, align the documentation with the strange definition of empty used by the
Tripp Lite state-of-charge approximation.