brcmfmac: conditionally compile firmware protocol source code

The host-interface can select which protocol implementation it
needs. Selecting PCIe will include the msgbuf protocol and selecting
USB and/or SDIO will include the bcdc protocol. The PCIe kconfig
option assures the dependencies for msgbuf are met, ie. HAS_DMA.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>