ath10k: re-disable interrupts after target init

If MSI isn't configured device ROM program expects
legacy interrupts to be enabled before it can
fully boot. Don't forget to disable legacy
interrupts after that.

While at it re-use the legacy irq enabling helper
instead of calling ath10k_pci_write32().

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>