mx25: fix spi device registration typo

Commit 35bab058 (ARM: imx: change the way
spi-imx devices are registered) contained a typo in mx25, leading to link time
failure.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>