PCI: dwc: ep: Implement capability search using PCI core APIs

The PCI core now provides generic PCI_FIND_NEXT_CAP() macros to search for
PCI capabilities, using config accessors we supply.

Use them in the DWC EP driver to implement dw_pcie_ep_find_capability()
instead of duplicating the algorithm.

Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250819145828.438541-1-18255117159@163.com