eth: phytmac: phytmac_ioctl: Return -EOPNOTSUPP by default

Fix follow error with clang-19:

drivers/net/ethernet/phytium/phytmac_main.c:1818:2: error: variable 'ret' is used uninitialized whenever switch default is taken [-Werror,-Wsometimes-uninitialized]
 1818 |         default:
      |         ^~~~~~~
drivers/net/ethernet/phytium/phytmac_main.c:1822:9: note: uninitialized use occurs here
 1822 |         return ret;
      |                ^~~
drivers/net/ethernet/phytium/phytmac_main.c:1799:9: note: initialize the variable 'ret' to silence this warning
 1799 |         int ret;
      |                ^
      |                 = 0
1 error generated.

Signed-off-by: WangYuli <[email protected]>

drivers/net/ethernet/phytium/phytmac_main.c

@@ -1816,6 +1816,7 @@ static int phytmac_ioctl(struct net_device *dev, struct ifreq *rq, int cmd)
 		break;
 #endif
 	default:
+		ret = -EOPNOTSUPP;
 		break;
 	}