brcmfmac: expose sdio internal counters in debugfs

The structure brcmf_sdio contains a number of counters that are useful
for debugging. These were not available in user-space. This patch
exposes them in debugfs under the filename 'counters'.

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>