staging: mt7621-pinctrl: replace ENOSYS with better fitting error code

This commit replaces ENOSYS return with ENOTSUPP silencing the
following checkpatch warning:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-pinctrl/pinctrl-rt2880.c

@@ -398,7 +398,7 @@ static int rt2880_pinmux_probe(struct platform_device *pdev)
 	struct device_node *np;
 
 	if (!rt2880_pinmux_data)
-		return -ENOSYS;
+		return -ENOTSUPP;
 
 	/* setup the private data */
 	p = devm_kzalloc(&pdev->dev, sizeof(struct rt2880_priv), GFP_KERNEL);