TELNETasp_PT.cc: Fix sign of integer variable

gcc 10.1.0 warns/errors about narrowing conversion:
"""
TELNETasp_PT.cc: In member function ‘void TELNETasp__PortType::TELNETasp__PT::recv_msg_from_client(int&)’:
TELNETasp_PT.cc:1659:9: error: narrowing conversion of ‘-1’ from ‘int’ to ‘unsigned int’ [-Wnarrowing]
 1659 |   case -1:{
      |         ^
make[1]: *** [Makefile:180: TELNETasp_PT.o] Error 1
"""

Signed-off-by: Pau Espin Pedrol <pespin@sysmocom.de>