arm, at91, mpddrc: fix typo in ddr2_init()

use the configure value for computing the ba_off value
not the value from the cr register. This leaded in a
wrong ram configuration on the upcoming corvus spl board
support.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>