brcmsmac: fix statistic counter update function

The 802.11 core statistics are retrieved from the core registers
but not stored. So the debug code was never triggered to give a
warning message on tx underruns or rx overflows. This patch fixes
this and assures the statistics are stored in the snapshot.

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>