ath10k: defer irq registration until hif start()

It's impossible to rely on disable_irq() and/or CE
interrupt masking with legacy shared interrupts.
Other devices sharing the same irq line may assert
it while ath10k is doing something that requires
no interrupts.

Irq handlers are now registered after all
preparations are complete so spurious/foreign
interrupts won't do any harm. The handlers are
unregistered when no interrupts are required (i.e.
during driver teardown).

This also removes the ability to receive FW early
indication (since interrupts are not registered
until early boot is complete). This is not mission
critical (it's more of a hint that early boot
failed due to unexpected FW crash) and will be
re-added in a follow up patch.

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