net: ipa: count actual work done in gsi_channel_poll()

There is an off-by-one problem in gsi_channel_poll().  The count of
transactions completed is incremented each time through the loop
*before* determining whether there is any more work to do.  As a
result, if we exit the loop early the counter its value is one more
than the number of transactions actually processed.

Instead, increment the count after processing, to ensure it reflects
the number of processed transactions.  The result is more naturally
described as a for loop rather than a while loop, so change that.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>