usb: gadget: f_fs: Fix kernel panic if use_os_string not set

If c->cdev->use_os_string flag is not set,
don't need to invoke ffs_do_os_descs() in _ffs_func_bind.
So uninitialized ext_compat_id pointer won't be accessed by
__ffs_func_bind_do_os_desc to cause kernel panic.

Signed-off-by: Jim Lin <jilin@nvidia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>