can: flexcan: flexcan_mailbox_read() make use of flexcan_write64() to mark the mailbox as read

In the previous patch the function flexcan_write64() was introduced.

This patch replaces the open coded variant in flexcan_mailbox_read()
that marks a mailbox as read, by a single call to flexcan_write64().

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