etrax: Update ethernet mgm-ctrl reg on writes

Some SW drivers dont keep track of what they've written and
depend on the HW latching write contents for later
read+modify+write sequences.
Signed-off-by: NEdgar E. Iglesias <edgar@axis.com>

hw/etraxfs_eth.c

@@ -437,6 +437,7 @@ eth_writel (void *opaque, target_phys_addr_t addr, uint32_t value)
 				eth_validate_duplex(eth);
 			}
 			eth->mdio_bus.mdc = !!(value & 4);
+			eth->regs[addr] = value;
 			break;
 
 		case RW_REC_CTRL: