bcma: make bcma_host_pci_(up|down) calls safe for every config

We were providing declarations but actual code was compiled only with
CONFIG_BCMA_HOST_PCI set. This could result in:
ERROR: "bcma_host_pci_down" [drivers/net/wireless/brcm80211/brcmsmac/brcmsmac.ko] undefined!
ERROR: "bcma_host_pci_up" [drivers/net/wireless/brcm80211/brcmsmac/brcmsmac.ko] undefined!
ERROR: "bcma_host_pci_down" [drivers/net/wireless/b43/b43.ko] undefined!
ERROR: "bcma_host_pci_up" [drivers/net/wireless/b43/b43.ko] undefined!

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>