libfdt: avoid shadowing "err" in FDT_RW_CHECK_HEADER

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

libfdt/fdt_rw.c: In function 'fdt_add_mem_rsv':
libfdt/fdt_rw.c:177:2: error: declaration of 'err' shadows a previous
local [-Werror=shadow]
libfdt/fdt_rw.c:175:6: error: shadowed declaration is here
[-Werror=shadow]
libfdt/fdt_rw.c: In function 'fdt_del_mem_rsv':
libfdt/fdt_rw.c:194:2: error: declaration of 'err' shadows a previous
local [-Werror=shadow]
libfdt/fdt_rw.c:192:6: error: shadowed declaration is here
[-Werror=shadow]
libfdt/fdt_rw.c: In function 'fdt_set_name':
...

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>