Small fixes and clean up

This is a small series which fix usbg_set_function_attrs()
which previously tried to set attributes which are read only
on configfs.

I have also went through code and marked attributes which should not
change during function calls with const qualifiers.

Third patch is a small enhancement which replace ether_aton() and
ether_ntoa() calls with their reentrant versions ether_aton_r() and
ether_ntoa().