net: rmnet_usb: Enable autosuspend

The call to enable autosuspend was recently removed from the
qcserial driver. Re-add it here in the RmNet driver as we know
this device will support it.

Change-Id: Iee1d1dbece01eaab99b03c64ebea0fd5ed87eafa
Signed-off-by: Jack Pham <jackp@codeaurora.org>