drivers: can: mcan: remove unused message marker split

The Bosch M_CAN Message Marker, an 8-bit field copied from Tx Buffer
elements to Tx Event FIFO elements to aid software in identifying Tx frame
status, need not be split into an index and a count, as the count is
unused.

Remove this split and replace "struct can_mcan_mm" with a simple "uint8_t".

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>