din: Split rpp_din_get() into two functions

din: Split rpp_din_get() into two functions

One for reading via SPI chip, one for reading via comparators with
programmable threshold.

One of the reasons for split are the return values of the functions.
SPI chip reports whether a switch connected to the DIN pin is open or
closed, which depends on the pin setup. Comparators return logical value,
which might be a negated value returned by the SPI chip.