PCI: imx6: Call dw_pcie_wait_for_link() from start_link() callback only when required

Since the DWC driver is already calling dw_pcie_wait_for_link() after
calling the start_link() callback, remove the redundant
dw_pcie_wait_for_link() call from imx_pcie_start_link(). It is still
required to call this function for controllers supporting Gen 2 and higher
link speeds.

Suggested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
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: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250416081314.3929794-3-hongxing.zhu@nxp.com