mtd: mtdswap: fix compilation warning

Fix the following compilation warning:

drivers/mtd/mtdswap.c: In function ‘mtdswap_pick_gc_eblk’:
drivers/mtd/mtdswap.c:859: warning: ‘idx’ may be used uninitialized in this function
drivers/mtd/mtdswap.c:859: note: ‘idx’ was declared here

Initialize idx to -1 because this value means "no tree choosen".
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/mtdswap.c

@@ -850,7 +850,7 @@ static int mtdswap_wlfreq(unsigned int maxdiff)
 static int mtdswap_choose_wl_tree(struct mtdswap_dev *d)
 {
 	static unsigned int pick_cnt;
-	unsigned int i, idx, wear, max;
+	unsigned int i, idx = -1, wear, max;
 	struct rb_root *root;
 
 	max = 0;