net: mdio-mux: bcm-iproc: fix wrong getter and setter pair

mdio_mux_iproc_probe() uses platform_set_drvdata() to store md pointer
in device, whereas mdio_mux_iproc_remove() restores md pointer by
dev_get_platdata(&pdev->dev). This leads to wrong resources release.

The patch replaces getter to platform_get_drvdata.

Fixes: 98bc865a ("net: mdio-mux: Add MDIO mux driver for iProc SoCs")
Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>