gve: fix dma sync bug where not all pages synced

The previous commit had a bug where the last page in the memory range
could not be synced. This change fixes the behavior so that all the
required pages are synced.

Fixes: 9cfeeb57 ("gve: Fixes DMA synchronization")
Signed-off-by: Adi Suresh <adisuresh@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>