usb: phy: phy-mxs-usb: set the correct platform drvdata

We need to set mxs_phy rather as the platform drvdata so that we can get
the correct mxs_phy in mxs_phy_remove().

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>