drivers: ixgbevf: fix unsigned underflow

'count' is unsigned. It is initialized to zero, then it can be increased
multiple times, and finally it is used in such a way:

   >>>> count--;
   |
   |    /* clear timestamp and dma mappings for remaining portion of packet */
   |    while (count >= 0) {
   |            count--;
   |            ...
   ^
If count is zero here (so, it was never increased), we would have a very
long loop :)

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>