work around bug in bnx2x firmware

This is an ugly kludge around a firmware bug.
* before this commit, disable-offload reports failure on bnx2x
* after  this commit, disable-offload reports success on bnx2x

bnx2x firmware has a bug that prevents the kernel from
disabling tx-checksumming. It appears to be fixed in
firmware 7.0.0 according to references [1] and [2],
both dated 2011-July-25:

    5. Problem: CQ57063 - tx checksum offload feature would not turn off when
                'ethtool -K eth<xxx> tx off' was used.

    Change:     Since at startup bnx2x declares support for ipv4 and ipv6
                checksum offload, connect the ethtool ipv6 set checksum premade
                op to the set_tx_csum op which affects both ipv4 and ipv6
                instead of the ipv4 op which affects only ipv4.

    Introduced: 7.0.0.

References:

[1] http://delivery04.dhe.ibm.com/sar/CMA/XSA/035pg/1/brcm_dd_nic_nx2-2.1.12c-1.70.40_rhel5_32-64.chg

[2] ftp://ftp.supermicro.com/CDR-X9_1.11_for_Intel_X9_platform/Broadcom/Server/Linux/Driver/RELEASE.bnx2x.TXT
Automatic commit of package [disable-offload] release [0.3-1].