Fix gcc warnings

Remove -Wunused-but-set-variable (`error`). Cast calls with
`(void)` to note that the return value is explicitly ignored.

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division

usr.sbin/bhyve/pci_passthru.c

@@ -361,7 +361,7 @@ msix_table_write(struct vmctx *ctx, int vcpu, struct passthru_softc *sc,
 	uint64_t *dest64;
 	size_t entry_offset;
 	uint32_t vector_control;
-	int error, index;
+	int index;
 
 	pi = sc->psc_pi;
 	if (offset >= pi->pi_msix.pba_offset &&
@@ -416,8 +416,8 @@ msix_table_write(struct vmctx *ctx, int vcpu, struct passthru_softc *sc,
 		/* If the entry is masked, don't set it up */
 		if ((entry->vector_control & PCIM_MSIX_VCTRL_MASK) == 0 ||
 		    (vector_control & PCIM_MSIX_VCTRL_MASK) == 0) {
-			error = vm_setup_pptdev_msix(ctx, vcpu,
-			    sc->psc_sel.pc_bus, sc->psc_sel.pc_dev, 
+			(void)vm_setup_pptdev_msix(ctx, vcpu,
+			    sc->psc_sel.pc_bus, sc->psc_sel.pc_dev,
 			    sc->psc_sel.pc_func, index, entry->addr,
 			    entry->msg_data, entry->vector_control);
 		}