ti: ethernet: cpdma: Use correct format for genpool_*

Now that we can compile davinci_cpdma.c on 64-bit hosts, we can see that
the format used for printing a size_t type is incorrect, use %zd
accordingly.

Fixes: aeec3021

 ("net: ethernet: ti: cpdma: remove used_desc counter")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>