stmmac: handle allocation errors in setup functions

If the allocations fail in either dwmac1000_setup() or dwmac100_setup()
then return NULL.  These are called from stmmac_mac_device_setup().  The
check for NULL returns in stmmac_mac_device_setup() needed to be moved
forward a couple lines.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>