UBI: Change the default percentage of reserved PEB

The actual value (1%) is too low for actual NAND devices, a huge
majority of device has 2% maximum bad blocks (SLC or MLC).
(Actually it's 20 blocks on a 1024 blocks device, 40/2048...)
Signed-off-by: NRichard Genoud <richard.genoud@gmail.com>

drivers/mtd/ubi/Kconfig

@@ -29,7 +29,7 @@ config MTD_UBI_WL_THRESHOLD
 
 config MTD_UBI_BEB_RESERVE
 	int "Percentage of reserved eraseblocks for bad eraseblocks handling"
-	default 1
+	default 2
 	range 0 25
 	help
 	  If the MTD device admits of bad eraseblocks (e.g. NAND flash), UBI