The ATmega PWM implementation only supports a maximum resolution of 256.
Thus when running `osci` with `OSC_STEPS` = 1000 an assertion will fail and
the test crashes, even though the PWM peripheral works perfectly fine.
Limit OSC_STEPS to 256 so it works on all platforms.
The LED flashing doesn't look any more coarse.
`UINT16_MAX` should be the same value on all platforms, so why not use
`puts` instead of `printf` here instead. Also, `llvm` detects an issue
with using the `UINT16_MAX` macro with `PRIu16` here.