i2c: mux: reg: don't log an error for probe deferral

It's possible that i2c_mux_reg_probe_dt() could return -EPROBE_DEFER.
In that case, driver will request a probe deferral and an error
suggesting device tree parsing problem will be reported. This is
a pretty confusing information. Let's change the error handling,
so driver will be able to request probe deferral without logging
not related errors.

Signed-off-by: Tomasz Bachorski <tomasz.bachorski@nokia.com>
Signed-off-by: Peter Rosin <peda@axentia.se>