net: axienet: move use of resource after validity check

We were accessing the pointer returned from platform_get_resource before
checking if it was valid, causing an oops if it was not. Move this access
after the call to devm_ioremap_resource which does the validity check.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>