xtensa iss_net_setup() must be __init

This patch fixes the following section mismatch:

<-- snip -->

...
MODPOST vmlinux.o
WARNING: vmlinux.o(.text+0x395c): Section mismatch: reference to .init.text:__alloc_bootmem (between '__invalidate_dcache_all' and '__simc')
...

<-- snip -->

---
4cefa581c6b3aa4a474516f8916fdad2a8fb233f

index b61fb36..506ac13 100644
@@ -746,7 +746,7 @@ struct iss_net_init {
 
 #define ERR KERN_ERR "iss_net_setup: "
 
-static int iss_net_setup(char *str)
+static int __init iss_net_setup(char *str)
 {
 	struct iss_net_private *device = NULL;
 	struct iss_net_init *new;