BaseTools/GenVtf: Provide string width in '%s' specifier in format string

BaseTools/GenVtf: Provide string width in '%s' specifier in format string

String width is not specified for '%s' specifier in the format string for
scanf functions.

This commit now specifies the string length for '%s' in format strings
according to the size of receiving buffers.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>