tests/periph_timer_short_relative_set: fix diff for non 32 bit timers

[MichelRottleuthner]

Contribution description

Fixes the problem described in #12610 (comment): the diff calculation between two timestamps is not working properly for timers that are not 32 bit.

Testing procedure

Look at the code, run /tests/periph_timer_short_relative_set on a platform with 16 bit timer (e.g. iotlab-m3).

Issues/PRs references

#10493, #12610

[kaspar030]

run /tests/periph_timer_short_relative_set on a platform with 16 bit timer (e.g. iotlab-m3).

Don't have such a board at hand, could someone else give this a quick run?

[aabadie]

platform with 16 bit timer (e.g. iotlab-m3).

IoT-LAB provides quite a few of them. Is it required to have direct access to hardware to test this ?

[MichelRottleuthner]

Physical access is not required so iot lab should do. Keep in mind that the expected result is that the tast will fail on that platform (refer to #12610 (comment)).

[cgundogan]

Tested on an iotlab-m3 node with a 16-bit timer. I adjusted the interval variable in main.c to compensate for the timer width and assure that a wrap-around happens. With interval=1000 the counter goes down to 951, while this patch let's the test run down to iteration 2. ACK