brcmsmac: expose 802.11 core statistics in debugfs

The 802.11 statistics obtained from the device can be retrieved
dumping the 'macstat' file in debugfs folder.

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