can: flexcan: remove not needed struct flexcan_priv::tx_mb and struct flexcan_priv::tx_mb_idx

The previous patch changes the TX path to always use the last mailbox
regardless of the used offload scheme (rx-fifo or timestamp based). This
means members "tx_mb" and "tx_mb_idx" of the struct flexcan_priv don't
depend on the offload scheme, so replace them by compile time constants.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>