s6gmac: Read buffer overflow

Check whether index is within bounds before testing the element.
In the last iteration i is PHY_MAX_ADDR. the condition
`!(p = pd->mii.bus->phy_map[PHY_MAX_ADDR])' is undefined and may
evaluate to false, which leads to a dereference of this invalid
phy_map in the phy_connect() below.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>