rtlwifi: Use pci_pcie_cap()

Use function pci_pcie_cap() instead of accessing struct member directly.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>