davinci_cpdma: make cpdma_chan_split_pool static

The cpdma_chan_split_pool() function is not used outside of
the driver, so make it static to avoid the following sparse
warning:

drivers/net/ethernet/ti/davinci_cpdma.c:725:5: warning: symbol 'cpdma_chan_split_pool' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>