Use the new DC_FIELD_STRING callback if it exists

This recognizes recognize some strigns (serial number and firmware
version), and the ones that it doesn't recognize it adds as extra data
using Dirk's new interface.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Cleanup liquivision.c

strndup doesn't exist on Windows.
Mark / remove unused variables.
Plus codying style cleanup.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>