spi: Add missing kerneldoc description for parameter

Commit ca5d2485 ("spi: Add THIS_MODULE to spi_driver in SPI core")
adds the new __spi_register_driver() function, but keeps the kerneldoc
for the spi_register_driver() function in place and forgets to add the
description for the new owner parameter.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>