TODO: fix the "=+"

[cederom]

As reported by @MemphisCZ in #6:

Hi @cederom

I using your lib in one of my project and found maybe a mistyping in your source code.
For example in file libswd_cmdq.c line 243:

res=libswd_drv_transmit(libswdctx, cmd);
if (res<0) return res;
cmdcnt=+res;

the cmdcnt will then hold only the last result of res not the sum of them, which will emit in some cases problems.
To fix all these kind of problems, maybe it would be better to find all "=+" across the libswd source code and change it to "+="

Best regards,
Tomas Kamenicky / @MemphisCZ