can: flexcan: put TX mailbox into TX_INACTIVE mode after tx-complete

commit de5944883ebbedbf5adc8497659772f5da7b7d72 upstream.

After sending a RTR frame the TX mailbox becomes a RX_EMPTY mailbox. To avoid
side effects when the RX-FIFO is full, this patch puts the TX mailbox into
TX_INACTIVE mode in the transmission complete interrupt handler. This, of
course, leaves a race window between the actual completion of the transmission
and the handling of tx-complete interrupt. However this is the best we can do
without busy polling the tx complete interrupt.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>