serial: sirf: transfer more bytes once to decrease interrupts

the current codes send 1 bytes, then after getting TX done interrupt,
send subsequent bytes. it causes redundant interrupts.
for example, if we have 3 bytes in TX buffer, the TX flow is:
1. send 1 byte
2. get TX down interrupt
3. send the left 2 bytes
4. get TX down interrupt

this patch moves to send more bytes and decrease interrupts, the new
flow is:
1. send 3 bytes
2. get TX down interrupt
Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sirfsoc_uart.c

@@ -290,7 +290,8 @@ static void sirfsoc_uart_start_tx(struct uart_port *port)
 	if (sirfport->tx_dma_chan)
 		sirfsoc_uart_tx_with_dma(sirfport);
 	else {
-		sirfsoc_uart_pio_tx_chars(sirfport, 1);
+		sirfsoc_uart_pio_tx_chars(sirfport,
+			SIRFSOC_UART_IO_TX_REASONABLE_CNT);
 		wr_regl(port, ureg->sirfsoc_tx_fifo_op, SIRFUART_FIFO_START);
 		if (!sirfport->is_marco)
 			wr_regl(port, ureg->sirfsoc_int_en_reg,

drivers/tty/serial/sirfsoc_uart.h

@@ -449,4 +449,4 @@ struct sirfsoc_uart_port {
 
 /* I/O Mode */
 #define SIRFSOC_UART_IO_RX_MAX_CNT		256
-#define SIRFSOC_UART_IO_TX_REASONABLE_CNT	6
+#define SIRFSOC_UART_IO_TX_REASONABLE_CNT	256