ath10k: fix NAPI enable/disable symmetry for AHB interface

Move NAPI enable to 'ath10k_ahb_hif_start' from
'ath10k_ahb_hif_power_up'. This is to maintain the symmetry
of calling napi_enable() from ath10k_ahb_hif_start() so that it
matches with  napi_disable() being called from ath10k_pci_hif_stop().

This change is based on the crash fix from Kalle for PCI interface in
commit 1427228d ("ath10k: fix napi crash during rmmod when probe
firmware fails").

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>