net: Declare physical address as phys_addr_t unsigned type

Use phys_addr_t instead of int for addresses.
Addresses can't be < 0.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/include/net.h b/include/net.h
index 18d279e..3da35fe 100644
--- a/include/net.h
+++ b/include/net.h
@@ -81,7 +81,7 @@
 struct eth_device {
 	char name[16];
 	unsigned char enetaddr[6];
-	int iobase;
+	phys_addr_t iobase;
 	int state;
 
 	int  (*init) (struct eth_device *, bd_t *);