samples: shields: lmp90100_evb: rtd: improve readability, fix off-by-one

Improve the readability of the LMP90100-EVB RTD sample by adding
comments and further definitions instead of magic values within the
code.

This furthermore fixes an off-by-one error for the maximum ADC value
used in the formula for calculating the resistance of the RTD (8388607
vs. 8388608).

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>