brcm80211: fmac: move module entry points to dhd_linux.c

The module_init/exit functions are moved to dhd_linux.c to prepare
for supporting multiple host interface types.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>