MINOR: mux-quic: set expect data only on frontend side

MINOR: mux-quic: set expect data only on frontend side

Stream data layer is notified that data is expected when FIN is
received, which marks the end of the HTTP request. This prepares data
layer to be able to handle the expected HTTP response.

Thus, this step is only relevant on frontend side. On backend side, FIN
marks the end of the HTTP response. No further content is expected, thus
expect data should not be set in this case.

Note that se_expect_data() invokation via qcs_attach_sc() is not
protected. This is because this function will only be called during
request headers parsing which is performed on the frontend side.