brcmfmac: remove debugfs functionality from wl_cfg80211.c

brcmfmac: remove debugfs functionality from wl_cfg80211.c

In wl_cfg80211.c debugfs directory was created to expose
dtim_period and beacon_interval. However, this can be easily
obtained using iw so it is removed from the driver.

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