[gn][xdc-server] Fix a few warnings in new gn build for xdc-server

[22813->1015/23885 ~58] CXX host-x64-linux-clang/obj/system/host/xdc-server/xdc-server.xdc-server.cpp.o
../system/host/xdc-server/xdc-server.cpp:82:30: warning: comparison of integers of different signs: 'ssize_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Wsign-compare]
        while (total_written < len_to_write) {
               ~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
../system/host/xdc-server/xdc-server.cpp:95:45: warning: implicit conversion loses integer precision: 'long' to 'int' [-Wshorten-64-to-32]
            int offset = transfer->offset() + res;
                ~~~~~~   ~~~~~~~~~~~~~~~~~~~^~~~~
../system/host/xdc-server/xdc-server.cpp:119:21: warning: implicit conversion loses integer precision: 'ssize_t' (aka 'long') to 'int' [-Wshorten-64-to-32]
            int n = recv(fd(), buf, UsbHandler::Transfer::MAX_WRITE_DATA_SIZE, 0);
                ~   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../system/host/xdc-server/xdc-server.cpp:276:37: warning: implicit conversion loses integer precision: 'std::__2::vector<pollfd, std::__2::allocator<pollfd> >::size_type' (aka 'unsigned long') to 'int' [-Wshorten-64-to-32]
        int num_sockets = poll_fds_.size();
            ~~~~~~~~~~~   ~~~~~~~~~~^~~~~~
4 warnings generated.

BLD-361 #comment [gn][xdc-server] Fix a few warnings in new gn build for xdc-server
ZX-3415 #comment [gn][xdc-server] Fix a few warnings in new gn build for xdc-server

Test: CQ
Change-Id: I35f975285dffd647e2bc7834a6c636b14809caab