spi: spi-mem: Fix a typo in the documentation of struct spi_mem

Fix a typo in the @drvpriv description.

Signed-off-by: Frieder Schrempf <frieder.schrempf@exceet.de>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>