Use @retval in mobilesync.c doxygen
Use @retval in mobilesync.c doxygen

When enumerating a list of possible return codes in doxygen,
these values can be listed using @retval instead of @return. This
gives a prettier output.
Since doing the conversion is tedious, I've only done mobilesync.c
so far.

Tatsh/libimobiledevice,patweb/libimobiledevice,libimobiledevice/libimobiledevice,patweb/libimobiledevice,upbit/libimobiledevice,kalev/libimobiledevice,kalev/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,upbit/libimobiledevice,kalev/libimobiledevice,libimobiledevice-win32/libimobiledevice,upbit/libimobiledevice,libimobiledevice/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice,patweb/libimobiledevice,Tatsh/libimobiledevice,Tatsh/libimobiledevice