brcmfmac: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in struct
brcm_if, use stats from struct net_device.  Also remove the now
unnecessary .ndo_get_stats function.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>