net: ethernet: ti: cpsw-phy-sel: check bus_find_device() ret value

This fixes klockworks warnings: Pointer 'dev' returned from call to
function 'bus_find_device' at line 179 may be NULL and will be dereferenced
at line 181.

    cpsw-phy-sel.c:179: 'dev' is assigned the return value from function 'bus_find_device'.
    bus.c:342: 'bus_find_device' explicitly returns a NULL value.
    cpsw-phy-sel.c:181: 'dev' is dereferenced by passing argument 1 to function 'dev_get_drvdata'.
    device.h:1024: 'dev' is passed to function 'dev_get_drvdata'.
    device.h:1026: 'dev' is explicitly dereferenced.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
[nsekhar@ti.com: add an error message, fix return path]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>