board/matrix_vision/mvblx/sys_eeprom.c: fix buffer overflow

Fix error detected by cppcheck:

[board/matrix_vision/mvblx/sys_eeprom.c:353]: (error) Buffer is
accessed out of bounds.

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Michael Jones <michael.jones@matrix-vision.de>
Acked-by: Michael Jones <michael.jones@matrix-vision.de>