PM / devfreq: rockchip-dfi: Fix an error message

There is a typo in the message. The clock name should be 'pclk_ddr_mon'.
Fix it.

While at it, switch to dev_err_probe() which is less verbose, filters
-EPROBE_DEFER, and log the error code in a human readable way.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>