PCI: imx6: Simplify ltssm_enable() by using ltssm_off and ltssm_mask

Add drvdata::ltssm_off and drvdata::ltssm_mask to simplify
imx6_pcie_ltssm_enable(disable)() logic.

Link: https://lore.kernel.org/r/20240220161924.3871774-5-Frank.Li@nxp.com


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>