[firmware/Uart] uart_putchar precondition needs to be more generic

[dayeol]

In UartProperties.v, uart_putchar has a precondition execution t IDLE which means that the state needs to be IDLE.
However, this isn't necessarily required as you can write to WDATA as long as the FIFO is not full (i.e., uart_tx_full returns 0). #888
https://github.com/project-oak/silveroak/pull/888/files/e4726f92f4731ac63f4e06d952f32f54863d99f1#diff-d9411a34f7afad9c8a02c00eddb68b70ce66fe2bb82e348da435351876b3fcb0