MLK-10392-2 flexcan: inactive all message buffer before enable can device

There may be possible valid and active messages in message buffer caused
by the random data in memory.
It will cause the CAN sends out the random message once it's up.

So we have to initialize the MB memory and inactive all message buffers before
enable the CAN device to avoid the accidently tranfser.

Signed-off-by: Dong Aisheng <b29396@freescale.com>