PCI: imx6: Add missing reference clock disable logic

PCI: imx6: Add missing reference clock disable logic

Ensure the *_enable_ref_clk() function is symmetric by addressing missing
disable parts on some platforms.

Fixes: d0a75c791f98 ("PCI: imx6: Factor out ref clock disable to match enable")
Link: https://lore.kernel.org/r/20241126075702.4099164-7-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>