hyperv/hn: Put debug message under bootverbose

MFC after:	1 week
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D7875

sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c

@@ -629,8 +629,10 @@ netvsc_attach(device_t dev)
 
 	ether_ifattach(ifp, eaddr);
 
+	if (bootverbose) {
 		if_printf(ifp, "TSO: %u/%u/%u\n", ifp->if_hw_tsomax,
 		    ifp->if_hw_tsomaxsegcount, ifp->if_hw_tsomaxsegsize);
+	}
 
 	/* Inform the upper layer about the long frame support. */
 	ifp->if_hdrlen = sizeof(struct ether_vlan_header);