tty/serial: atmel: add comment for the ring buffer size macro

There is a macro named ATMEL_SERIAL_RINGSIZE which suggesting that it
corresponds to the real size of the ring buffer. Let warn people that
there is a factor of four since allocation size is
sizeof(struct atmel_uart_char) * ATMEL_SERIAL_RINGSIZE.
Signed-off-by: NLudovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/atmel_serial.c

@@ -108,6 +108,12 @@ struct atmel_uart_char {
 	u16		ch;
 };
 
+/*
+ * Be careful, the real size of the ring buffer is
+ * sizeof(atmel_uart_char) * ATMEL_SERIAL_RINGSIZE. It means that ring buffer
+ * can contain up to 1024 characters in PIO mode and up to 4096 characters in
+ * DMA mode.
+ */
 #define ATMEL_SERIAL_RINGSIZE 1024
 
 /*