m68k: use strscpy() to instead of strncpy()

The implementation of strscpy() is more robust and safer.

Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>