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>