[PATCH] clear warnings about compare signed-unsigned values

This just removes few annoying warnings. About the conversion of string to signed int. - dev_index = boost::lexical_cast< unsigned int >( hackrf_serial ); + dev_index = boost::lexical_cast< int >( hackrf_serial ); hackrf_device_list_open checks if value of dev_index is out of range (eg negative) and returns proper error. In future negative value can be used for something so this check is not necessary nor wanted. P