VW: swap specific distance value checks for truthiness checks

[elkoled]

goal: working distance buttons regardless of car-specific inconsistencies

VW's distance buttons generally have three possible values.

• a value of 1 is in theory distance -1
• a value of 2 is in theory distance +1
• a value of 3 is in theory distance toggle

the actual behavior varies. I know that some PQ cars send a value of 1 when distance is pressed depending on equipment. Many cars will send a value of 3, but never use 1 or 2 even when manually adjusting distance. Presumably these inconsistencies could surface across the entire VW lineup.

Since openpilot doesn't really care about distance +1 vs -1 vs toggle, I find the cleanest fix here is to simply check the truthiness of the signal, rather than a specific value.

a side note: why include 2? I've included 2 here for consistency. If a vehicle actually did use values 1 and 2, I don't want DIST+ and DIST- to be treated differently