brcmfmac: fix watchdog timer regression

The watchdog timer is used to put the device in a low-power mode when
it is idle for some time. This timer is stopped during that mode and
should be restarted upon activity. This has been broken by commit
d4150fced0365 ("brcmfmac: Simplify watchdog sleep."). This patch
restores the behaviour as it was before that commit.

Reported-by: Pontus Fuchs <pontusf@broadcom.com>
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: Kalle Valo <kvalo@codeaurora.org>