usb: gadget: f_fs: Add the missing get_alt callback

The Apple CarLife iAP gadget has a descriptor in userspace with two
alternate settings. The host sends the set_alt request to configure
alt_setting 0 or 1, and this is verified by the subsequent get_alt
request.

This patch implements and sets the get_alt callback. Without the
get_alt callback, composite.c abruptly concludes the
USB_REQ_GET/SET_INTERFACE request, assuming only one alt setting
for the endpoint.

unlike the uvc and ncm, f_fs gadget is fully implemented in userspace,
and driver just reset the eps and generate the event. so no additional
adaptaion associated with this change is not required in set_alt callback

Signed-off-by: Hardik Gajjar <hgajjar@de.adit-jv.com>
Link: https://lore.kernel.org/r/20240301124708.120394-1-hgajjar@de.adit-jv.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>