mtd: nandsim: bugfix: fail if overridesize is too big

If override size is too big, the module was actually loaded instead of
failing, because retval was not set.

This lead to memory corruption with the use of the freed structs nandsim
and nand_chip.

Cc: stable@vger.kernel.org
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>