stmmac: fix check for phydev being open

Current check of phydev with IS_ERR(phydev) may make not much sense
because of_phy_connect() returns NULL on failure instead of error value.

Still for checking result of phy_connect() IS_ERR() makes perfect sense.

So let's use combined check IS_ERR_OR_NULL() that covers both cases.

Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>