brcmsmac: fix TSSI idle estimation

The baseband multiplier must be zero during TSSI idle estimation
and restored afterwards.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Tested-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>