bge: Workaround BCM5906 chip bug

bge: Workaround BCM5906 chip bug

When auto-negotiation results in half-duplex operation, excess collision
on the ethernet link may cause internal chip delays that may result in
subsequent valid frames being dropped due to insufficient receive buffer
resources.  The workaround is to choose de-pipeline method as a flow
control decision for SDI.  De-pipeline method allows only 1 data in TxMbuf
at a time such that a request to RDMA from SDI is made only when TxMbuf
is empty.

Obtained-from: FreeBSD 214219