mtd: orion_nand: Fix build failure caused by typo

Commit e9903060 ("mtd: orion_nand.c: add 
error handling and use resource_size()") introduced a build error -- it 
assigns something to a undeclared variable 'err', whereas the rest of 
the code uses 'ret' for this task.

This patch fixes this typo and thus removes the build failure.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>