drivers: can: mcan: reintroduce assert in can_mcan_send()

Reintroduce the assert on put_idx being within the limit in can_mcan_send()
but move it to just after the value was determined and add a comment on its
purpose.

With the current code, this will never get triggered due to the tx_sem
being acquired before (meaning a free TX slot was acquired).

Avoid assigning a signed initial literal to an unassigned type.

Fixes: 8dc4dea11243556f282926236fd76f31f17d1033

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