Fix the last serious bug I've been chasing:
Fix the last serious bug I've been chasing:

The DP83820/83821 has an undocumented limitation concerning jumbo frames
and TX checksum offload. In order for TX checksum offload to work, the
outgoing frame must fit entirely within the TX FIFO, which is 8192 bytes
in size. This isn't a problem, until you try to send a 9000-byte frame,
at which point the TX DMA engine goes to sleep. It turns out that if
you want to send a jumbo frame larger than 8170 bytes (8192 - 64), you
have to turn off the TX checksum support.

As a workaround, I changed nge_ioctl() so that if the user selects an
MTU larger than 8152 bytes, we clear the if_hwassist flags. The flags
will be set again once the MTU is reduced to a smaller value.

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