PCI: dwc: Use private data pointer of "struct irq_domain" to get pcie_port

PCI: dwc: Use private data pointer of "struct irq_domain" to get pcie_port

No functional change. Get "struct pcie_port *" from private data
pointer of "struct irq_domain" in dw_pcie_irq_domain_free() to make
it look similar to how "struct pcie_port *" is obtained in
dw_pcie_irq_domain_alloc()

Link: https://lore.kernel.org/r/20191220100550.777-1-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>