xen: Code style conformity

xen: Code style conformity

Cleaning up the following:

ERROR: do not use assignment in if condition
#281: FILE: drivers/xen/pvblock.c:260:
+       if ((err = xenbus_switch_state(XBT_NIL, nodename,
CHECK:COMPARISON_TO_NULL: Comparison to NULL could be written "err"
#52: FILE: drivers/xen/pvblock.c:298:
+       if (err != NULL) {
ERROR: do not use assignment in if condition
#176: FILE: drivers/xen/gnttab.c:103:
+               if ((flags = nflags) & (GTF_reading | GTF_writing)) {
WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
#329: FILE: include/xen/gnttab.h:1:
+/*
WARNING: Misplaced SPDX-License-Identifier tag - use line 1 instead
#330: FILE: include/xen/gnttab.h:2:
+ * SPDX-License-Identifier: GPL-2.0
ERROR: do not use assignment in if condition
#630: FILE: lib/sscanf.c:558:
+                               if ((n = inr) < width) {

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Reviewed-by: Simon Glass <sjg@chromium.org>