board/renesas/ecovec/ecovec.c: fix buffer overflow

Fix error detected by cppcheck:

[board/renesas/ecovec/ecovec.c:66]: (error) Buffer is accessed out of
bounds.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>