Restore part of old behaviour of bge_tick() -- if link is up, then don't

Restore part of old behaviour of bge_tick() -- if link is up, then don't
call mii_tick().  Mii_tick() will read and write various PHY registers.
For bge(4), PHY accessing registers normally involves flipping AUTOPOLL
bit of BGE_MI_MODE, which trigers interrupts about link state changing.

# This changing restores netperf UDP_RR