can: flexcan: introduce struct flexcan_priv::tx_mask and make use of it

The current driver uses FLEXCAN_IFLAG2_MB() to generate the mask to check for
the TX complete interrupt. This works well, as the driver will always use the
last mailbox for TX, which falls into the iflag2 register.

To support CANFD the payload size has to increase to 64 bytes and the
number of mailboxes will decrease so much that the TX mailbox will be
handled in the iflag1 register.

This patch introduces a tx_mask in the struct flexcan_priv (similar to rx_mask)
and makes use of it. The actual support to handle the TX mailbox in iflag1 will
be added in the next patches.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>