serial: 8250_of: use of_property_read_bool()

Use slightly more compact of_property_read_bool() calls for the boolean
properties instead of of_find_property() calls.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>