drivers: can: mcp2515: reduce the number of tx buffers to 1

Reduce the number of TX buffers in use on the MCP2515 from 3 to 1 in
order to avoid CAN frame priority inversion.

The MCP2515 is unable to do internal TX frame arbitration based on the
CAN-ID of the frame. Priority must be set per TX buffer and the priority
cannot be rewritten unless the frame transmission is aborted.

Fixes: #26541

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