PCI: j721e: correct lane count mask based on lane numbers

Platforms that don't have 4x support have a single bit mask, and mask
should based on the number of lanes that can be configured.

Fixes: 719b08dc

 ("PCI: j721e: Add PCIe 4x lane selection support")
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Matt Ranostay <mranostay@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>