serial: MIPS: lantiq: fix clock error check

The clock should be checked with the proper IS_ERR() api before using it.

Signed-off-by: John Crispin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/lantiq.c

···
 	struct ltq_uart_port *ltq_port = to_ltq_uart_port(port);
 	int retval;
 
-	if (ltq_port->clk)
+	if (!IS_ERR(ltq_port->clk))
 		clk_enable(ltq_port->clk);
 	port->uartclk = clk_get_rate(ltq_port->fpiclk);
 
···
 		port->membase + LTQ_ASC_RXFCON);
 	ltq_w32_mask(ASCTXFCON_TXFEN, ASCTXFCON_TXFFLU,
 		port->membase + LTQ_ASC_TXFCON);
-	if (ltq_port->clk)
+	if (!IS_ERR(ltq_port->clk))
 		clk_disable(ltq_port->clk);
 }