When including the header in decklink_enc.cpp it would be fed
through the C++ compiler rather than the C compiler, which has
more strict warnings when comparing signed/unsigned values.

Make the local variables unsigned to match the arguments they are
being passed for those functions.
Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>