tty/serial: at91: use of_property_read_bool

Use of_property_read_bool to check for the existence of a property.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e1,e2;
statement S2,S1;
@@
-       if (of_get_property(e1,e2,NULL))
+       if (of_property_read_bool(e1,e2))
        S1 else S2
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>