net: davinci_mdio: Fix sparse warning

This patch fixes following sparse warning
davinci_mdio.c:85:27: warning: symbol 'default_pdata' was not declared. Should it be static?
Also makes the default_pdata as a constant.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>