drivers: can: stm32_bxcan: fix CAN mailbox overwrite

Fixes overwriting a mailbox before sending it completes.
Overwriting can occur when one thread has waited for
another thread to complete the send function and
fills a mailbox selected based on
the transmit_status_register variable,
which has not been updated after waiting and,
accordingly, does not contain up-to-date information
about the mailboxes used.

Signed-off-by: Mikhail Siomin <victorovich.01@mail.ru>