saul: Ignore extra dimensions in read functions

[bergzand]

Contribution description

This PR modifies the current ADC and GPIO SAUL functions to not fill the extra dimensions of the phydat_t struct with zeros. This allows the calling function to automatically detect the dimensions used by the SAUL sensor or actuator by filling it with PHYDAT_MIN or PHYDAT_MAX and checking which of the phydat::val[]s are modified after a saul_reg_read().

This PR is a small cleanup in the ADC and GPIO SAUL functions to not fill the unused dimensions of the phydat_t struct with zeros`

This PR can be ignored if there is a better way to automatically detect the number of dimensions used by a sensor (without hard coding).

The return value of the read call indicates the number of dimensions

@haukepetersen does this PR align with the concepts and ideas behind SAUL?

Testing procedure

examples/saul should still read out ADC and GPIO-based SAUL devices (with saul_gpio and saul_adc included as modules).

Issues/PRs references

None

[smlng]

doesn't the return value of the function state how many dimensions are used? However, I agree none the less that changing (or setting) values that are actually not used in a buffer passed into the function is not so nice behaviour.

[bergzand]

doesn't the return value of the function state how many dimensions are used? However, I agree none the less that changing (or setting) values that are actually not used in a buffer passed into the function is not so nice behaviour.

Well, that's a bit embarrassing, so much for reading the docs.

Let's consider this a cleanup then, I'll modify the description to reflect this

[smlng]

tested ACK

[smlng]

@leandrolanzieri any objections?

[leandrolanzieri]

@leandrolanzieri any objections?

Nope, as cleanup this makes sense :-) ACK