ath10k: make sure to really disable irqs

This fixes two corner cases.

One is a race between disabling copy engine
interrupts and unhandled pending interrupts on the
host. This could end up with a runaway tasklet and
consequently memory leak of a few copy engine
rx buffers.

The other one is an unexpected (and non-maskable
via device CSR) MSI fw indication interrupt during
teardown. This could trigger the same problem as
the first corner case.

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