facetimehd: Only unload module if it is loaded

The pre-sleep service exits if any command fails. Unloading facetimehd
without it being loaded blocks subsequent commands from running.

Note: `modprobe -r` works a bit better when unloading unused modules,
and is preferrable to `rmmod`. However, the facetimehd module does not
support suspending. In this case, it seems preferable to forcefully
unload the module. `modprobe` does not support a `--force` flag when
removing, so we are left with `rmmod`.

See:
 - https://github.com/NixOS/nixpkgs/pull/14883
 - https://github.com/patjak/bcwc_pcie/wiki#known-issues