PCI: dwc: Replace magic number by defines

Replace magic numbers by a self-explained define to ease human
comprehension.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>