can: flexcan: fix flexcan_chip_start() on imx6

In the flexcan_chip_start() function first the flexcan core is going through
the soft reset sequence, then the RX FIFO is enabled.

With the hardware is put into FIFO mode, message buffers 1...7 are reserved by
the FIFO engine. The remaining message buffers are in reset default values.
This patch removes the bogus initialization of the message buffers, as it
causes an imprecise external abort on imx6.

Cc: linux-stable <stable@vger.kernel.org>
Reported-by: Lothar Waßmann <LW@KARO-electronics.de>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>