tty: ipwireless: fix error handling

ipwireless_send_packet() can only return 0 on success and -ENOMEM on
error, the caller should check non zero for error condition

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20200821161942.36589-1-ztong0001@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>