[netdrvr] usb/hso: Cleanup rfkill error handling

Yup, this appears to be the problem, thanks. I think &hso_net->net->dev
is more intuitive for the error message, so I've used that. I've also
added missing line endings on the error messages and set our local
rfkill structure element to NULL on failure so we don't try to call
rfkill_unregister on driver removal if we failed to register at all.

The patch below Works For Me (TM); the device is detected fine, can be
removed without problems and connects ok. I'll have a prod at why the
rfkill stuff isn't working next, but I believe this cleanup of the error
handling is appropriate no matter what the issue with registration is.

Signed-Off-By: Jonathan McDowell <noodles@earth.li>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>