USB: usb_wwan: remove comment from close

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: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>