bcma: add bcma_core_pci_config_fixup()

This code is based on code from pcie_misc_config_fixup() in brcmsmac.
This patch is part of the move of pci specific code from brcmsmac to
bcma.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>