spi/mpc8xxx: don't use __exit_p to wrap plat_mpc8xxx_spi_remove

The function plat_mpc8xxx_spi_remove is defined using __devexit, so don't
use __exit_p but __devexit_p to wrap it.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>