stmmac: change the stmmac_dvr_probe return type to int

Since stmmac_dvr_probe takes care of setting driver data and
assign resources to the priv structure there is no need to
access the priv structure from the other probe functions.
This mean that this function can be changed into just return
an int and thus simplifying the callers.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>