PCI: imx6: Convert DIRECT_SPEED_CHANGE quirk code to use a flag

Both i.MX7D and i.MX8MQ have the same behaviour when it comes to
clearing DIRECT_SPEED_CHANGE bit when no speed change occurs, so to
handle variants correctly add a flag instead of checking the IP block
variant.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
[lorenzo.pieralisi@arm.com: updated log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: "A.s. Dong" <aisheng.dong@nxp.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>