[netdrvr s2io] make debug_level variable static

Fixes 'make allyesconfig' build conflict with infiniband, which
also has a global variable named 'debug_level'.

drivers/net/s2io.h

@@ -61,7 +61,7 @@ typedef enum xena_max_outstanding_splits {
 #define	INTR_DBG	4
 
 /* Global variable that defines the present debug level of the driver. */
-int debug_level = ERR_DBG;	/* Default level. */
+static int debug_level = ERR_DBG;	/* Default level. */
 
 /* DEBUG message print. */
 #define DBG_PRINT(dbg_level, args...)  if(!(debug_level<dbg_level)) printk(args)