ath10k: split up pci irq code

Hardware waits until host signals whether it has
chosen MSI(-X) or shared legacy interrupts. It is
not required for the driver to register interrupt
handlers immediately.

This patch prepares the pci irq code for more
changes.
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>