usb: gadget: legacy: fix redundant initialization warnings

Fix the following cppcheck warnings:

drivers/usb/gadget/legacy/inode.c:1364:8: style: Redundant initialization for 'value'. The initialized value is overwritten$
 value = -EOPNOTSUPP;
       ^
drivers/usb/gadget/legacy/inode.c:1331:15: note: value is initialized
 int    value = -EOPNOTSUPP;
              ^
drivers/usb/gadget/legacy/inode.c:1364:8: note: value is overwritten
 value = -EOPNOTSUPP;
       ^
drivers/usb/gadget/legacy/inode.c:1817:8: style: Redundant initialization for 'value'. The initialized value is overwritten$
 value = -EINVAL;
       ^
drivers/usb/gadget/legacy/inode.c:1787:18: note: value is initialized
 ssize_t   value = len, length = len;
                 ^
drivers/usb/gadget/legacy/inode.c:1817:8: note: value is overwritten
 value = -EINVAL;
       ^
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>