serial: max310x: Fix tx_empty() callback

BugLink: https://bugs.launchpad.net/bugs/1855313

[ Upstream commit a8da3c78 ]

Function max310x_tx_empty() accesses the IRQSTS register, which is
cleared by IC when reading, so if there is an interrupt status, we
will lose it. This patch implement the transmitter check only by
the current FIFO level.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/serial/max310x.c

@@ -763,12 +763,9 @@ static void max310x_start_tx(struct uart_port *port)
 
 static unsigned int max310x_tx_empty(struct uart_port *port)
 {
-	unsigned int lvl, sts;
+	u8 lvl = max310x_port_read(port, MAX310X_TXFIFOLVL_REG);
 
-	lvl = max310x_port_read(port, MAX310X_TXFIFOLVL_REG);
-	sts = max310x_port_read(port, MAX310X_IRQSTS_REG);
-
-	return ((sts & MAX310X_IRQ_TXEMPTY_BIT) && !lvl) ? TIOCSER_TEMT : 0;
+	return lvl ? 0 : TIOCSER_TEMT;
 }
 
 static unsigned int max310x_get_mctrl(struct uart_port *port)