[MTD] FTL Fix missing pointer assignment

For the case that mtd partitions are enabled it would cause a 0-pointer 
dereferencing in mtdpart.c:mtd_erase_callback()

Signed-off-by: Herbert Valerio Riedel <hvr@inso.tuwien.ac.at>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>