usb: phy-mv-usb: constify attribute_group structures.

attribute_group are not supposed to change at runtime. All functions
working with attribute_group provided by <linux/sysfs.h> work with
const attribute_group. So mark the non-const structs as const.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/phy/phy-mv-usb.c

@@ -650,7 +650,7 @@ static struct attribute *inputs_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group inputs_attr_group = {
+static const struct attribute_group inputs_attr_group = {
 	.name = "inputs",
 	.attrs = inputs_attrs,
 };