mtd: tests: don't use mtd0 as a default

mtd tests may erase the mtd device, so force the user to specify which
mtd device to test by using the module parameter. Disable the default
(using mtd0) since this may destroy a vital part of the flash if the
module is inserted accidently or carelessly.

Reported-by: Roland Kletzing <devzero@web.de>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@intel.com>