brcmfmac: create debugfs files for bus-specific layer

Since we moved the drivers debugfs directory under ieee80211 debugfs the
debugfs entries need to be added after wiphy_register() has been called.
For most part that has been done accordingly, but for the debugfs entries
added by SDIO it was not and failed silently. This patch fixes that by
adding a bus-layer callback for it.

Fixes: 856d5a01 ("brcmfmac: allocate struct brcmf_pub instance using wiphy_new()")
Reported-by: Russel King <linux@armlinux.org.uk>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>