ath6kl: Refactor ath6kl_init_hw_start() and ath6kl_init_hw_stop()

So that these functions will be used to re-initialize the fw
upon detecting fw error. This refactoring moves ar->state
setting out of core stop/start functionality.

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