es519xx.c: Fix a few compiler warnings (clang).

es519xx.c: Fix a few compiler warnings (clang).

  CC       libsigrok_hw_common_dmm_la-es519xx.lo
es519xx.c:632:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:659:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:688:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:717:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:746:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:773:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
es519xx.c:800:33: warning: missing field 'is_voltage' initializer
      [-Wmissing-field-initializers]
        struct es519xx_info info = { 0 };
                                       ^
7 warnings generated.