Fix two warnings recently introduced

[julianoes]

Found by AppleClang:

/Users/julianoes/src/MAVSDK/src/core/system_impl.cpp:1384:66: warning: self-comparison always evaluates to true [-Wtautological-compare]
    return _parent.get_base_mode() & (MAV_MODE_FLAG_SAFETY_ARMED == MAV_MODE_FLAG_SAFETY_ARMED);

And GCC:

/home/julianoes/src/MAVSDK/src/plugins/mission_raw_server/mission_raw_server_impl.cpp:315:26: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<mavsdk::MAVLinkMissionTransfer::ItemInt>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
  315 |     if (_current_seq + 1 == _current_mission.size()) {
      |         ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

[bazfp]

D:\a\MAVSDK\MAVSDK\src\core\system_impl.cpp(1384,39): error C2220: the following warning is treated as an error [D:\a\MAVSDK\MAVSDK\build\release\src\core\mavsdk.vcxproj]
D:\a\MAVSDK\MAVSDK\src\core\system_impl.cpp(1384,39): warning C4806: '&': unsafe operation: no value of type 'bool' promoted to type 'MAV_MODE_FLAG' can equal the given constant 

Oopsie! bool MavsdkImpl::get_base_mode() const

This should be uint8_t