ath10k: move napi_enable to hif_start for consistent pairing

There is unbalanced napi_{enable,disable}() behavior as they
are being called from hif_snoc_{power_up/stop).

The fix is to call napi_enable() from ath10k_snoc_hif_start()
so that it matches with napi_disable() being called from
ath10k_snoc_hif_stop().

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>