AdsTool: replace SYSTEMSERVICE_OPENGENERIC with FOPEN::READ
AdsTool: replace SYSTEMSERVICE_OPENGENERIC with FOPEN::READ

Both have the same value and clang 11.0.1-2 in c++2a mode will complain
with:
../AdsTool/main.cpp:253:69: error: bitwise operation between different enumeration types ('bhf::ads::nSystemServiceOpenFile' and 'bhf::ads::FOPEN') is deprecated [-Werror,-Wdeprecated-enum-enum-conversion]
                                bhf::ads::SYSTEMSERVICE_OPENGENERIC | bhf::ads::FOPEN::READ | bhf::ads::FOPEN::BINARY |
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~
../AdsTool/main.cpp:279:75: error: bitwise operation between different enumeration types ('bhf::ads::nSystemServiceOpenFile' and 'bhf::ads::FOPEN') is deprecated [-Werror,-Wdeprecated-enum-enum-conversion]
        AdsFile::Delete(device, next, bhf::ads::SYSTEMSERVICE_OPENGENERIC | bhf::ads::FOPEN::ENABLE_DIR);
                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Previously, we blindly adopted the usage of SYSTEMSERVICE_OPENGENERIC
from generic documentation. Looking closer it seems okay to replace
it with FOPEN::READ.

Signed-off-by: Patrick Bruenn <27dd71a6a4a167461f53ec06010a2a08d399a527@beckhoff.com>

Beckhoff/ADS,Beckhoff/ADS,Beckhoff/ADS,Beckhoff/ADS