brcm80211: fmac: remove duplicate statistics from driver data structure

Some dongle statistics are stored in two places and synced when net
device status inquired. There is no need to do it this way any more.
Direct all usage to dongle stats structure in order to increase
readability.

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