net: stmmac: Fix the logic of checking if RX Watchdog must be enabled

RX Watchdog can be disabled by platform definitions but currently we are
initializing the descriptors before checking if Watchdog must be
disabled or not.

Fix this by checking earlier if user wants Watchdog disabled or not.

Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>