PCI: cadence: Update cdns_pcie_writel() function signature

cdns_pcie_writel() writes a long value; change the value parameter type
from u16 to u32 to rectify the function signature and related behaviour.

Signed-off-by: Alan Douglas <adouglas@cadence.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>