net: ipa: clear pending interrupts before enabling

We enable the completion interrupt for channel or event ring
commands only when we issue them.  The interrupt is disabled after
the interrupt has fired, or after we have timed out waiting for it.

If we time out, the command could complete after the interrupt has
been disabled, causing a state change in the channel or event ring.
The interrupt associated with that state change would be delivered
the next time the completion interrupt is enabled.

To avoid previous command completions interfering with new commands,
clear all pending completion interrupts before re-enabling them for
a new command.

Fixes: b4175f87 ("net: ipa: only enable GSI event control IRQs when needed")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>