Revert "nforce: Disable MTU changes."

This reverts commit 25742b78fb.

It was confirmed the MTU change introduction was unrelated to the
problems seen in this ticket.

src/add-ons/kernel/drivers/network/ether/nforce/dev/nfe/if_nfe.c

@@ -1722,7 +1722,6 @@ nfe_ioctl(if_t ifp, u_long cmd, caddr_t data)
 	error = 0;
 	init = 0;
 	switch (cmd) {
-#ifndef __HAIKU__
 	case SIOCSIFMTU:
 		if (ifr->ifr_mtu < ETHERMIN || ifr->ifr_mtu > NFE_JUMBO_MTU)
 			error = EINVAL;
@@ -1742,7 +1741,6 @@ nfe_ioctl(if_t ifp, u_long cmd, caddr_t data)
 			}
 		}
 		break;
-#endif
 	case SIOCSIFFLAGS:
 		NFE_LOCK(sc);
 		if (if_getflags(ifp) & IFF_UP) {