net: stmmac: add sanity check to device_property_read_u32_array call

Currently the call to device_property_read_u32_array is not error checked
leading to potential garbage values in the delays array that are then used
in msleep delays.  Add a sanity check to the property fetching.

Addresses-Coverity: ("Uninitialized scalar variable")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>