MFC: r281337

Don't enable RX and TX before their initial configuration is done, i. e.
after setting up interrupt moderation but before turning interrupts on.
This matches what Realtek's r8168 Linux driver does as of version 8.039.00
and fixes problems with certain incarnations of certain MAC revisions
like the interface requiring an extra up/down-cycle after boot to start
working or DMA configuration not being adhered to.

PR: 193743, 197535