r295133 attempted to deactivate TSO in the 100Mbit link case with this
adapter to work around bugs in TSO handling at this speed.

em_init_locked is called during first boot of the adapter and will
see that link_speed is unitialized, effectively turning off tso for
all cards at all speeds, which I believe was *not* the intent.

Move the handling of TSO deactivation to the link handler where we can
more effectively make the decision about what to do. In addition,
completely purge the TSO capabilities instead of disabling just CSUM_TSO.

Thanks to jhb for explanation of the hw capabilites api.

Thanks to royger and cognet for testing the 100Mbit failure case to
ensure that their adapters do indeed still work.

MFC after: 1 week
Sponsored by: Limelight Networks