Remove superfluous and cryptic comment from close.

It should be obvious that we're balancing the autopm_put in open (and
that operation already mentions the autopm_get done in the USB serial
core).
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>