CAN write possible bug

[CAN::write()](https://github.com/arduino/ArduinoCore-renesas/blob/16bdbea34732678faa950858a7d9e63e621778b4/libraries/Arduino_CAN/src/R7FA4M1_CAN.cpp#L256) uses mailbox 16 if the ID is standard, which is configured as a receive mailbox. I think this [should be 8](https://github.com/arduino/ArduinoCore-renesas/blob/16bdbea34732678faa950858a7d9e63e621778b4/libraries/Arduino_CAN/src/R7FA4M1_CAN.cpp#L77)?

I think this works because the FSP only checks if the [TRMREQ flag is set](https://github.com/renesas/fsp/blob/5e8fbb10c752e16272790bd3a2587485d0377835/ra/fsp/src/r_can/r_can.c#L537), which it won't be for a receive mailbox, and then configures it for transmit.

I'm guessing that all future receives will come via mailbox 17 onwards, as 16 is no longer configured as a receive, which would break any filters assigned to 16.

Start in libraries/Arduino_CAN/src/R7FA4M1_CAN.cpp at CAN::write() around line 256, then compare the mailbox setup around line 77 with the FSP transmit handling in ra/fsp/src/r_can/r_can.c around line 537. Verify whether using mailbox 16 changes receive-mailbox allocation or filters assigned to it, and confirm that standard-ID writes and subsequent receives use the intended mailboxes.

Written by the indexing model from the issue text.