ptp: Fix dp83640 build warning when building statically

If the dp83640 driver is not built as a module, the
MODULE_DEVICE_TABLE reference to dp83640_tbl nops out.

Since the table isn't referenced elsewhere, it it causes
the following warning:
drivers/net/phy/dp83640.c:1095: warning: ‘dp83640_tbl’ defined but not used

This apparently is common with mdio_device_id table structures,
and is avoided by using __maybe_unused annotation.
Signed-off-by: John Stultz <john.stultz@linaro.org>

drivers/net/phy/dp83640.c

@@ -1092,7 +1092,7 @@ MODULE_LICENSE("GPL");
 module_init(dp83640_init);
 module_exit(dp83640_exit);
 
-static struct mdio_device_id dp83640_tbl[] = {
+static struct mdio_device_id __maybe_unused dp83640_tbl[] = {
 	{ DP83640_PHY_ID, 0xfffffff0 },
 	{ }
 };