i2c: fix always-true condition in i2c_probe_chip()

Per dm_i2c_ops.probe_chip documentation, i2c_probe_chip() shall fallback
to default probe method when .probe_chip() returns -ENOSYS.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Reviewed-by: Heiko Schocher <hs@denx.de>