Input: sentelic - silence some underflow warnings

I have a static checker that complains when we check for an upper bound
but don't have a corresponding check for a lower bound.  With this code,
the upper bound check seems not really required, so it is not a bug to
leave the lower bound check out as well.  But let's silence the warning
by making these variables unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>