drivers: can: mcan: manually track available TX buffers

The MCAN driver operates in TX queue mode (TXBC.TFQM = 1). In this mode
TXFQS.TFQPI returns the first available buffer (usually buffer zero).

Hardware is free to re-use a buffer as soon as TX completes, it does not
have to wait for the matching TX event to be processed.

If a TX completes and that TX buffer is re-used before processing the TX
event, two TX events for the same buffer occur. The first event calls the
second events TX callback, and the second event results in a NULL pointer
exception.

In a "normal" configuration, the TX event ISR will always preempt the
queuing of a TX frame to the same TX buffer.
However, this issue could occur if:
 * Sending a message with ISRs temporarily disabled.
 * The ISR is processed on a different core to the TX call.

The fix is to manually track which TX buffers are available, only freeing
a buffer after the TX event has been processed.

The MCAN user manual states that this is allowed:
"The application may use register TXBRP instead of the Put Index and may
place messages to any Tx Buffer without pending transmission request"

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>