can: flexcan: fix NULL pointer exception during bringup

Commit cbffaf7a ("can: flexcan: Always use last mailbox for TX")
introduced a loop letting i run up to (including) ARRAY_SIZE(regs->mb)
and in the body accessed regs->mb[i] which is an out-of-bounds array
access that then resulted in an access to an reserved register area.

Later this was changed by commit 0517961c ("can: flexcan: Add
provision for variable payload size") to iterate a bit differently but
still runs one iteration too much resulting to call

	flexcan_get_mb(priv, priv->mb_count)

which results in a WARN_ON and then a NULL pointer exception. This
only affects devices compatible with "fsl,p1010-flexcan",
"fsl,imx53-flexcan", "fsl,imx35-flexcan", "fsl,imx25-flexcan",
"fsl,imx28-flexcan", so newer i.MX SoCs are not affected.

Fixes: cbffaf7a

 ("can: flexcan: Always use last mailbox for TX")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: linux-stable <stable@vger.kernel.org> # >= 4.20
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>