i2c: mux: reg: use of_property_read_bool()

Use more compact of_property_read_bool() calls for the boolean properties
instead of of_find_property() calls in i2c_mux_reg_probe_dt().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Peter Rosin <peda@axentia.se>