Fix warnings reported by Clang static code analyzer:

    CC      hw/net/rocker/rocker.o
  hw/net/rocker/rocker.c:213:9: warning: Value stored to 'tx_tso_mss' is never read
          tx_tso_mss = rocker_tlv_get_le16(tlvs[ROCKER_TLV_TX_TSO_MSS]);
          ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/net/rocker/rocker.c:217:9: warning: Value stored to 'tx_tso_hdr_len' is never read
          tx_tso_hdr_len = rocker_tlv_get_le16(tlvs[ROCKER_TLV_TX_TSO_HDR_LEN]);
          ^                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/net/rocker/rocker.c:255:9: warning: Value stored to 'tx_l3_csum_off' is never read
          tx_l3_csum_off += tx_tso_mss = tx_tso_hdr_len = 0;
          ^                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: dc488f88
Reported-by: Clang Static Analyzer
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200217101637.27558-1-philmd@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>