[ppc64] llan: fix fatal kernel trap when system is low on memory

When running several builders in parallel, on QEMU, with 8GB of
memory, a fatal kernel trap (0x300 (data storage interrupt))
caused by llan driver is sometimes observed, when the system
starts to run out of swap space.

This happens because, at llan_intr(), a phyp call to add a
logical LAN buffer is always made when llan_add_rxbuf() fails,
even if it fails to allocate a new buffer.

PR:	235489
Reviewed by:	jhibbits
Differential Revision:	https://reviews.freebsd.org/D19084

sys/powerpc/pseries/phyp_llan.c

@@ -386,8 +386,6 @@ llan_intr(void *xsc)
 		/* llan_add_rxbuf does DMA sync and unload as well as requeue */
 		if (llan_add_rxbuf(sc, rx) != 0) {
 			if_inc_counter(sc->ifp, IFCOUNTER_IERRORS, 1);
-			phyp_hcall(H_ADD_LOGICAL_LAN_BUFFER, sc->unit,
-			    rx->rx_bufdesc);
 			continue;
 		}