wlcore: fix error handling in wlcore_event_fw_logger

wlcore_read/wlcore_write can return negative values so it should
be assigned to signed variable.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2120705

Fixes: 3719c17 ("wlcore/wl18xx: fw logger over sdio")
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ti/wlcore/event.c

@@ -38,7 +38,7 @@
 
 int wlcore_event_fw_logger(struct wl1271 *wl)
 {
-	u32 ret;
+	int ret;
 	struct fw_logger_information fw_log;
 	u8  *buffer;
 	u32 internal_fw_addrbase = WL18XX_DATA_RAM_BASE_ADDRESS;