USB: usbtmc: fix stupid bug in open()

open() will never succeed, as we always return -ENODEV.  Fix this
obvious bug.

Thanks to Jouni Ryno for reporting it.

Reported-by: Jouni Ryno <Jouni.Ryno@fmi.fi>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>