sh_eth: use PIR_* bits

sh_mdio_init() uses the bare numbers instead of the PHY interface bits, despite
these are declared in sh_eth.h as 'enum PIR_BIT'...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>