sys/phydat: fix phydat_to_json precondition

[LP-HAW]

Contribution description

RIOT kernel panics when calling phydat_to_json with dim=3:

sys/phydat/phydat_json.c:46 => 0x8049c14
*** RIOT kernel panic:
FAILED ASSERTION.

*** halted.

The precondition/assertion for parameter 'dim' in phydat_to_json is to restrictive. Calling phydat_to_json with dim=3 should be valid as can be seen in the documentation of phydat_to_json itself:
https://doc.riot-os.org/group__sys__phydat.html#gad7a5978e5119e3976addffd2891c76fa

This pull request allows phydat_to_json to be called with dim=3.

Testing procedure

Just call phydat_to_json with dim=3. For example:

#include <stdio.h>
#include <phydat.h>

int main(void)
{
    char json[100];
    phydat_t data = {0};
    phydat_to_json(&data, 3, json);
    puts(json);
}

Expected output would be:

{"d":[0,0,0],"u":"undefined"}

Issues/PRs references

(no references)

[chrysn]

Thanks, good catch and straightforward fix.

[maribu]

Congratulations and thanks for your first PR :)