ath5k: always calculate ANI listen time

Calculate 'listen' time also when automatic ANI is off, since this and the
"busy" time is useful information also in manual mode.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>