Increase EM_MAX_SCATTER to 64 such that the size of em_xmit()::segs[EM_MAX_SCATTER]
doesn't get overrun by things like NFS that can and do shove more than 32 segs when
being used with em(4) and TSO4.
Update tso handling code in em_xmit() with update from jhb@ in email thread:
set ifp->if_hw_tsomax, ifp->if_hw_tsomaxsegcount & ifp->if_hw_tsomaxsegsize
to appropriate values.
Define a TSO workaround "magic" number of 4 that is used to avoid an
alignment issue in hardware.
Change a couple of integer values that were used as booleans to actual
Ensure that em_enable_intr() enables the appropriate mask of interrupts
and not just a hardcoded define of values.