Change adc sysfs dir and increase size of MAX_BUF

ADC values are now retrieved from the IIO sysfs abstractions, rather than
from the adc helper. The new range is 0 to 4095. The size of MAX_BUF had to be increased
to accomodate the *large* filepath.
Change ADC channel filename

The ADC filenames now reflect the changes made earlier, with ADC values now
being obtained from the IIO sysfs entries instead of the adc helper entries.
Change scaling algorithm in analogRead

The new scaling algorithm is more correct, as suggested by Matt Porter
and Andrew Bradford. It scales down from (0 to 4095) to (0 to 1023).