ath10k: fix operating irq mode for ahb device

Earlier when operating irq mode is legacy, interrupts are disabled
and re-enabled based on num_msi_intrs. commit cfe9011a ("ath10k:
remove MSI range support") replaced num_msi_intrs by oper_irq_mode.
Since oper_irq_mode is not initialized for ahb devices (i.e qca4019),
device boot up is failed during probe.

Fixes: cfe9011a ("ath10k: remove MSI range support")
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>