usb: gadget: gr_udc: remove some unneeded error handling

Debugfs function return an ERR_PTR if they compiled out.  We don't need
to test for that here because if the debugfs file are compiled out then
it is ok to pass an ERR_PTR to debugfs_create_file() since it will just
be a no-op stub.

Debugfs return NULLs on error, but we don't need to test for that either
because debugfs_create_file() will accept NULL pointers.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>