serial: 8250: Revert "tty: serial: 8250_core: read only RX if there is something in the FIFO"

This reverts commit 0aa525d118.

The conditional RX-FIFO read seems to cause spurious interrupts and we
see just:
|serial8250: too much work for irq29

The previous behaviour was "default" for decades and Marvell's 88f6282 SoC
might not be the only that relies on it. Therefore the Omap fix is
reverted for now.

Fixes: 0aa525d118 ("tty: serial: 8250_core: read only RX if there is
something in the FIFO")
Reported-By: Nicolas Schichan <nschichan@freebox.fr>
Debuged-By: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -2138,7 +2138,7 @@ int serial8250_do_startup(struct uart_port *port)
 	/*
 	 * Clear the interrupt registers.
 	 */
-	if (serial_port_in(port, UART_LSR) & UART_LSR_DR)
+	serial_port_in(port, UART_LSR);
 	serial_port_in(port, UART_RX);
 	serial_port_in(port, UART_IIR);
 	serial_port_in(port, UART_MSR);
@@ -2300,7 +2300,7 @@ dont_test_tx_en:
 	 * saved flags to avoid getting false values from polling
 	 * routines or the previous session.
 	 */
-	if (serial_port_in(port, UART_LSR) & UART_LSR_DR)
+	serial_port_in(port, UART_LSR);
 	serial_port_in(port, UART_RX);
 	serial_port_in(port, UART_IIR);
 	serial_port_in(port, UART_MSR);
@@ -2394,7 +2394,6 @@ void serial8250_do_shutdown(struct uart_port *port)
 	 * Read data port to reset things, and then unlink from
 	 * the IRQ chain.
 	 */
-	if (serial_port_in(port, UART_LSR) & UART_LSR_DR)
 	serial_port_in(port, UART_RX);
 	serial8250_rpm_put(up);