spi: loopback: fix typo in MODULE_PARM_DESC

There should be an 's' on "dump_message" so it matches the module_param.

Fixes: 84e0c4e5 ('spi: add loopback test driver to allow for spi_master regression tests')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>