net: ethernet: ti: davinci_cpdma: make function cpdma_desc_pool_create static

The function cpdma_desc_pool_create is local to the source and does not
need to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'cpdma_desc_pool_create' was not declared. Should it
be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>