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>