net: ipa: fix an incorrect assignment

I spotted an error in a patch posted this week, unfortunately just
after it got accepted.  The effect of the bug is that time-based
interrupt moderation is disabled.  This is not technically a bug,
but it is not what is intended.  The problem is that a |= assignment
got implemented as a simple assignment, so the previously assigned
value was ignored.

Fixes: edc6158b ("net: ipa: define fields for event-ring related registers")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>