PCI: dwc: Fix dw_pcie_ep_find_capability() to return correct capability offset

commit beb4641a ("PCI: dwc: Add MSI-X callbacks handler") while
adding MSI-X callback handler, introduced dw_pcie_ep_find_capability()
and __dw_pcie_ep_find_next_cap() for finding the MSI and MSIX capability.

However if MSI or MSIX capability is the last capability (i.e there are
no additional items in the capabilities list and the Next Capability
Pointer is set to '0'), __dw_pcie_ep_find_next_cap will return '0'
even though MSI or MSIX capability may be present because of
incorrect ordering of the "next_cap_ptr" check. Fix it.

Fixes: beb4641a ("PCI: dwc: Add MSI-X callbacks handler")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>