brcmsmac: read PCI vendor and device id only for PCI devices

If brcmsmac is used on non PCI(s) devices it should not try to access
bus->host_pci.

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>