tests/driver_adt7310: Add explicit cast required for llvm

[fhessel]

Contribution description

This PR is related to PR #11352, where I suggest to add support for Linux' /dev/spidev devices to the native cpu. Enabling SPI causes the tests/driver_adt7310 test to be run with the llvm toolchain for the first time.

llvm doesn't allow the implicit cast to int in tests/driver_adt7310 (see Murdock's output).

This PR fixes this by using an explicit cast. Furthermore, abs() is replaced with labs(), as the parameter may take values which exceed a 16 bit int and could cause undefined behavior.

Testing procedure

Run BOARD=native TOOLCHAIN=llvm make -C tests/driver_adt7310 all without and with this PR and verify that the warning disappears. The build will still fail though, as native misses the required SPI support introduced in #11352.

Issues/PRs references

Required for PR #11352

[benpicco]

Only changes a cast to avoid generating a warning.
No change in output.

Actually the output was buggy before and would always print xx.0000001 C

Now the output is correct.