bcma: Prevent build of PCI host features in module

Attempting to build bcma.ko with BCMA_DRIVER_PCI_HOSTMODE=y results in
a build error due to use of symbols not exported from vmlinux:

ERROR: "pcibios_enable_device" [drivers/bcma/bcma.ko] undefined!
ERROR: "register_pci_controller" [drivers/bcma/bcma.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92: __modpost] Error 1

To prevent this, don't allow the host mode feature to be built if
CONFIG_BCMA=m

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>