can: flexcan: fix transition from and to freeze mode in chip_{,un}freeze

This patch factors out freeze and unfreeze of the CAN core into seperate
functions. Experiments have shown that the transition from and to freeze mode
may take several microseconds, especially the time entering the freeze mode
depends on the current bitrate.

This patch adds a while loop which polls the Freeze Mode ACK bit (FRZ_ACK) that
indicates a successfull mode change. If the function runs into a timeout a
error value is returned.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>