PCI: imx6: Turn off regulator when system is in suspend mode

The driver should undo any enables it did itself. The regulator disable
shouldn't be basing decisions on regulator_is_enabled().

Move the regulator_disable to the suspend function, turn off regulator when
the system is in suspend mode.

Link: https://lore.kernel.org/r/1657783869-19194-11-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>