wireless: wext: use struct iwreq earlier in the call chain

wireless: wext: use struct iwreq earlier in the call chain

To make it clear that we never use struct ifreq, cast from it
directly in the wext entrypoint and use struct iwreq from there
on. The next patch will remove the cast again and pass the
correct struct from the beginning.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>