sc16is7xx: use LSR_TEMT_BIT in .tx_empty()

LSR_TEMT_BIT (LSR bit 6) provides us exactly the information
we need to determine if transmission is finished - FIFO level
and shift register empty.  We can save ourselves reading FIFO
level explicitly if we use this bit.
Signed-off-by: NJakub Kicinski <kubakici@wp.pl>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sc16is7xx.c

@@ -683,12 +683,11 @@ static void sc16is7xx_start_tx(struct uart_port *port)
 
 static unsigned int sc16is7xx_tx_empty(struct uart_port *port)
 {
-	unsigned int lvl, lsr;
+	unsigned int lsr;
 
-	lvl = sc16is7xx_port_read(port, SC16IS7XX_TXLVL_REG);
 	lsr = sc16is7xx_port_read(port, SC16IS7XX_LSR_REG);
 
-	return ((lsr & SC16IS7XX_LSR_THRE_BIT) && !lvl) ? TIOCSER_TEMT : 0;
+	return (lsr & SC16IS7XX_LSR_TEMT_BIT) ? TIOCSER_TEMT : 0;
 }
 
 static unsigned int sc16is7xx_get_mctrl(struct uart_port *port)