usb: gadget: f_fs: handle control requests in config 0

Introduces a new FunctionFS descriptor flag named
FUNCTIONFS_CONFIG0_SETUP.

When this flag is enabled, FunctionFS userspace drivers can process
non-standard control requests in configuration 0.

Signed-off-by: Felix Hädicke <felixhaedicke@web.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>