net, phy, cpsw: fix NULL pointer deference

if phy_connect() did not find a phy, phydev is NULL and
following code in cpsw_phy_init() crashes. Fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Tom Rini <trini@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>