usb: gadget: function: f_fs: Demote function header which is clearly not kerneldoc

No attempt has been made to document the demoted function here.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/gadget/function/f_fs.c:2361: warning: Function parameter or member 'type' not described in '__ffs_data_do_os_desc'
 drivers/usb/gadget/function/f_fs.c:2361: warning: Function parameter or member 'h' not described in '__ffs_data_do_os_desc'
 drivers/usb/gadget/function/f_fs.c:2361: warning: Function parameter or member 'data' not described in '__ffs_data_do_os_desc'
 drivers/usb/gadget/function/f_fs.c:2361: warning: Function parameter or member 'len' not described in '__ffs_data_do_os_desc'
 drivers/usb/gadget/function/f_fs.c:2361: warning: Function parameter or member 'priv' not described in '__ffs_data_do_os_desc'

Cc: Felipe Balbi <balbi@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200706133341.476881-12-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -2352,7 +2352,7 @@ static int __must_check ffs_do_os_descs(unsigned count,
 	return _len - len;
 }
 
-/**
+/*
  * Validate contents of the buffer from userspace related to OS descriptors.
  */
 static int __ffs_data_do_os_desc(enum ffs_os_desc_type type,