brcmfmac: add stub version of brcmf_debugfs_get_devdir()

In case of compiling driver without DEBUG expose a stub function to make
writing debug code much simpler (no extra conditions). This will allow
e.g. using debugfs_create_file() without any magic if or #ifdef.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>