Revert "niu: Add support for byte queue limits."

This reverts commit efa230f2.

BQL doesn't work with how this driver currently only takes TX
interrupts every 1/4 of the TX ring.  That behavior needs to be fixed,
but that's a larger non-trivial task and for now we have to revert
BQL support as this makes the device currently completely unusable.

Signed-off-by: David S. Miller <davem@davemloft.net>