hw: megasas: return -1 when 'megasas_map_sgl' fails

The caller of 'megasas_map_sgl' will only check if the return
is zero or not. If it return 0 it means success, as in the next
patch we will consider 'iov_count=0' is an error, so let's
return -1 to indicate a failure.

Signed-off-by: Li Qiang <liq3ea@163.com>
Message-Id: <20200815141940.44025-2-liq3ea@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/megasas.c

@@ -280,7 +280,7 @@ static int megasas_map_sgl(MegasasState *s, MegasasCmd *cmd, union mfi_sgl *sgl)
     if (iov_count > MEGASAS_MAX_SGE) {
         trace_megasas_iovec_sgl_overflow(cmd->index, iov_count,
                                          MEGASAS_MAX_SGE);
-        return iov_count;
+        return -1;
     }
     pci_dma_sglist_init(&cmd->qsg, PCI_DEVICE(s), iov_count);
     for (i = 0; i < iov_count; i++) {
@@ -310,7 +310,7 @@ static int megasas_map_sgl(MegasasState *s, MegasasCmd *cmd, union mfi_sgl *sgl)
     return 0;
 unmap:
     qemu_sglist_destroy(&cmd->qsg);
-    return iov_count - i;
+    return -1;
 }
 
 /*