BUG/MEDIUM: h3: fix MAX_PUSH_ID handling

BUG/MEDIUM: h3: fix MAX_PUSH_ID handling

MAX_PUSH_ID frames are emitted by the client only on the control stream.
These conditions are checked via h3_check_frame_valid() since the
following patch.

  e4a5a64198bb084eaef2e71bfde65704a5db3931
  BUG/MINOR: h3: reject server MAX_PUSH_ID frame

However control stream test was inverted by mistake. This patch fixes
it.

Due to this bug, H3 connections were improperly closed on error by
haproxy for clients which send MAX_PUSH_ID frames. This has been
detected on the QUIC interop with aioquic and neqo clients.

This must be backported up to 3.3.