Standard AFHDS2A protocol transmits all values as 2 raw bytes and a stock FS-i6 transmitter shows voltage type values properly as signed. This change interprets all UNIT_VOLTS FlySky telemetry items as being signed 16-bit values so their display matches the values displayed on stock FS-i6 and FS-i6X receivers. Signed-off-by: Bryan Mayland <bmayland@capnbry.net>