Update vh400.c

Matched high-end voltages more closely with VWC for the piecewise function.

2.2a + b = 50.014  (match 2.2v with a VWC of 50.014)
3a + b = 100       (match 3.0v with a VWC of 100.0) 

.8a = 49.986       (linear combination)

a = 62.4825        (solve for a)
b = -87.4475       (solve for b)

Also, assume voltages higher than 3v are really just 100.0 VWC (mine gives some noise, and a 100% reading will actually be 3.02v, for example).