USB: serial: sierra driver send_setup() autopm fix

This patch presents a fix for the autosuspend feature implementation in
sierra usb serial driver for function sierra_send_setup().  Because it
is possible to call sierra_send_setup() before sierra_open() or after
sierra_close() we added a get/put interface activity to assure that the
usb control can happen even when the device is autosuspended.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Tested-by: Matthew Safar <msafar@sierrawireless.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>