usb: gadget: f_mass_storage: Remove runtime async resume

Currently the f_mass_storage function driver is calling
usb_gadget_autopm_get_async() in set_alt which increments
dwc3 power usage count which can prevent the controller to
transition into LPM during cable disconnect or composition
switch. Fix this removing the get_async/put_async from
function driver and let gadget driver take care of this.

Change-Id: I6dfb4ae1e4a9366ef56ebf40ea517f98b20250ee
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>