wl18xx: fix bogus compile warning on cc config option

Initialize val to 0, to remove the following warning with
CONFIG_CC_OPTIMIZE_FOR_SIZE. The compiler used was gcc 4.4.1
(Sourcery G++ Lite 2010q1-202).

drivers/net/wireless/ti/wl18xx/io.c: In function 'wl18xx_top_reg_read':
drivers/net/wireless/ti/wl18xx/io.c:57: warning: 'val' may be used uninitialized in this function
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>

drivers/net/wireless/ti/wl18xx/io.c

@@ -54,7 +54,7 @@ int wl18xx_top_reg_write(struct wl1271 *wl, int addr, u16 val)
 
 int wl18xx_top_reg_read(struct wl1271 *wl, int addr, u16 *out)
 {
-	u32 val;
+	u32 val = 0;
 	int ret;
 
 	if (WARN_ON(addr % 2))