tty: ipwireless, cleanup TIOCGSERIAL

In ipwireless_get_serial_info, struct serial_struct is memset to 0 and
then some members set to 0 explicitly.

Remove the latter as it is obviously superfluous.

And remove the retinfo check against NULL. copy_to_user will take care
of that.

Part of hub6 cleanup series.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jiri Kosina <jikos@kernel.org>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>