can: m_can_platform: remove not needed casts to struct m_can_plat_priv *

The struct m_can_classdev::device_data is a void pointer, so there's no
need to cast it to struct m_can_plat_priv *, when assigning the struct
m_can_plat_priv pointer.

This patch removes the not needed casts from the m_can_platform driver.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>