MODULE_DEVICE_TABLE only expands to something if it's compiled
for a module.  So when building-in support for the phys, the
mdio_device_id tables are unused.  Marking them with __maybe_unused
fixes the following warnings:

	drivers/net/phy/bcm63xx.c:134: warning: 'bcm63xx_tbl' defined but not used
	drivers/net/phy/broadcom.c:933: warning: 'broadcom_tbl' defined but not used
	drivers/net/phy/cicada.c:162: warning: 'cicada_tbl' defined but not used
	drivers/net/phy/davicom.c:222: warning: 'davicom_tbl' defined but not used
	drivers/net/phy/et1011c.c:114: warning: 'et1011c_tbl' defined but not used
	drivers/net/phy/icplus.c:137: warning: 'icplus_tbl' defined but not used
	drivers/net/phy/lxt.c:226: warning: 'lxt_tbl' defined but not used
	drivers/net/phy/marvell.c:724: warning: 'marvell_tbl' defined but not used
	drivers/net/phy/micrel.c:234: warning: 'micrel_tbl' defined but not used
	drivers/net/phy/national.c:154: warning: 'ns_tbl' defined but not used
	drivers/net/phy/qsemi.c:141: warning: 'qs6612_tbl' defined but not used
	drivers/net/phy/realtek.c:82: warning: 'realtek_tbl' defined but not used
	drivers/net/phy/smsc.c:257: warning: 'smsc_tbl' defined but not used
	drivers/net/phy/ste10Xp.c:135: warning: 'ste10Xp_tbl' defined but not used
	drivers/net/phy/vitesse.c:195: warning: 'vitesse_tbl' defined but not used

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>