serial: 8250_dw: use UPF_* constants when define flags

The flags member has upf_t type and corresponding macros to define them. This
patch converts ASYNC_SKIP_TEST to UPF_SKIP_TEST in 8250_dw.c.

Otherwise we got a sparse warning:
drivers/tty/serial/8250/8250_dw.c:302:46: warning: restricted upf_t degrades to integer
drivers/tty/serial/8250/8250_dw.c:302:62: warning: restricted upf_t degrades to integer
drivers/tty/serial/8250/8250_dw.c:302:26: warning: incorrect type in assignment (different base types)
drivers/tty/serial/8250/8250_dw.c:302:26:    expected restricted upf_t [usertype] flags
drivers/tty/serial/8250/8250_dw.c:302:26:    got unsigned int
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dw.c

@@ -267,7 +267,7 @@ static int dw8250_probe_of(struct uart_port *p,
 		p->membase += 7;
 #endif
 		p->serial_out = dw8250_serial_out_rb;
-		p->flags = ASYNC_SKIP_TEST | UPF_SHARE_IRQ | UPF_FIXED_TYPE;
+		p->flags = UPF_SKIP_TEST | UPF_SHARE_IRQ | UPF_FIXED_TYPE;
 		p->type = PORT_OCTEON;
 		data->usr_reg = 0x27;
 		has_ucv = false;