libfdt: avoid shadowing "err" in FDT_CHECK_HEADER

FDT_CHECK_HEADER declares an internal variable named "err" whose name is
far too generic and will produce the following -Wshadow warnings:

libfdt/fdt_ro.c: In function 'fdt_node_offset_by_compatible':
libfdt/fdt_ro.c:555:2: error: declaration of 'err' shadows a previous
local [-Werror=shadow]
libfdt/fdt_ro.c:553:14: error: shadowed declaration is here
[-Werror=shadow]
cc1: all warnings being treated as errors

Since this variable is only used internally in the macro, rename to
__err which should be prefixed enough not to cause new shadow warnings.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>