PCI: imx6: Use enum instead of bool for variant indicator

Use enumerated type instead of a boolean flag to specify the variant of
the PCIe IP block (6Q, 6SX, etc).  This patch has zero functional impact,
however it makes the code easier to extend for the case of more than 2
possible variants of an IP block (of which there are).

[bhelgaas: rewrap comment, remove extra blank line]
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>