mtd: nand: Use the mirror BBT descriptor when reading its version

The code responsible for reading the version of the mirror bbt was
incorrectly using the descriptor of the main bbt.

Pass the mirror bbt descriptor to 'scan_read_raw' when reading the
version of the mirror bbt.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org [v2.6.37+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>