hard-coded ADC path does not work with mainline kernel

I'm running a mainline Linux kernel on my BeagleBone blue. rc_battery_monitor fails because it hard-codes the path to the ADC as `/sys/bus/iio/devices/iio:device0`, however, in the mainline kernel it is at `/sys/bus/iio/devices/iio:device1`.

An easy but fragile fix would be to check both paths. A more robust fix would be to use libudev to find the correct path.

Background: Now that the counter subsystem has landed in the mainine kernel, I am considering writing a driver for the quadrature encoders, which is why I am using that kernel instead of the bb.org one.