drivers: usb: gadget: ether: Fix compiler warning

Latest gcc 6.2 compiler is throwing the below warning for am335x_baltos_defconfig
drivers/usb/gadget/ether.c:501:17: warning: 'mdlm_detail_desc' defined but not used [-Wunused-const-variable=]
 static const u8 mdlm_detail_desc[] = {

Guard mdlm_detail_desc with CONFIG_USB_ETH_SUBSET to avoid the warning

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>

drivers/usb/gadget/ether.c

@@ -507,6 +507,7 @@ static const struct usb_cdc_mdlm_desc mdlm_desc = {
  * can't really use its struct.  All we do here is say that we're using
  * the submode of "SAFE" which directly matches the CDC Subset.
  */
+#ifdef CONFIG_USB_ETH_SUBSET
 static const u8 mdlm_detail_desc[] = {
 	6,
 	USB_DT_CS_INTERFACE,
@@ -516,6 +517,7 @@ static const u8 mdlm_detail_desc[] = {
 	0,	/* network control capabilities (none) */
 	0,	/* network data capabilities ("raw" encapsulation) */
 };
+#endif
 
 #endif