ath6kl: Add support to detect fw error through heart beat

This patch adds support to detect fw error condition by sending
periodic message (heart beat challenge) to firmware. Upon reception
of the message, fw would send a response event to driver. When
there are no reponses from fw for about 5 cmd driver would
trigger the recovery logic assuming that fw has gone into an
error state.

Capable fw will advertise this capability through
ATH6KL_FW_CAPABILITY_HEART_BEAT_POLL bit. This feature
is disabled by default, can be enabled through a modparam
(heart_beat_poll). This modparam also confiures the polling
interval in msecs.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>