cxgb4: fix signed wrap around when decrementing index idx

Change predecrement compare to post decrement compare to avoid an
unsigned integer wrap-around comparison when decrementing idx in
the while loop.

For example, when idx is zero, the current situation will
predecrement idx in the while loop, wrapping idx to the maximum
signed integer and cause out of bounds reads on rxq_info->msix_tbl[idx].

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>