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>