PCI: imx6: Skip link up workaround for newer platforms

The current link setup procedure has one workaround to detect the devices
behind PCIe switches on some i.MX6 platforms. But this workaround is not
needed on recent i.MX7 platforms. So skip the workaround for platforms that
do not set the flag and start LTSSM directly.

Also, change the flag name from IMX_PCIE_FLAG_IMX_SPEED_CHANGE to
IMX_PCIE_FLAG_SPEED_CHANGE_WORKAROUND to match the usecase.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[mani: subject and description rewording]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://patch.msgid.link/20250416081314.3929794-2-hongxing.zhu@nxp.com