brcm80211: fmac: make brcmf_net_attach() static

The function brcmf_net_attach() is only used within dhd_linux.c so
it does not need to be extern. This patch makes the function static.

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>