usb: ehci-mx6: Move fdtdec_get_alias_seq() inside the CONFIG_MX6

On a imx7s-warp board the fdtdec_get_alias_seq() function
always fails.

As priv->portnr is only used on i.MX6, move fdtdec_get_alias_seq()
inside the CONFIG_MX6 block.

Signed-off-by: Fabio Estevam <festevam@denx.de>