PCI: mt7621: Remove unused function pcie_rmw()

PCI: mt7621: Remove unused function pcie_rmw()

Function pcie_rmw() is not being used at all and can be deleted. Hence get
rid of it, which fixes this warning:

  drivers/pci/controller/pcie-mt7621.c:112:20: warning: unused function 'pcie_rmw' [-Wunused-function]

Fixes: cf20a91d0588 ("PCI: mt7621: Add MediaTek MT7621 PCIe host controller driver")
Link: https://lore.kernel.org/r/20220124113003.406224-3-sergio.paracuellos@gmail.com
Link: https://lore.kernel.org/all/202201241754.igtHzgHv-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>