pinctrl: realtek: Fix return value and silence log for unsupported configs

pinctrl: realtek: Fix return value and silence log for unsupported configs

Treating unsupported configurations as errors causes upper layers (like the
GPIO subsystem) to interpret optional features as hard failures, aborting
operations or printing unnecessary error logs.

For example, during gpiod_get(), the GPIO framework attempts to set
PIN_CONFIG_PERSIST_STATE. Since this driver does not support it, false
error reports are generated in dmesg.

Fix this by returning -ENOTSUPP and demoting the log level to dev_dbg.

Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Tzuyi Chang <tychang@realtek.com>
Signed-off-by: Yu-Chun Lin <eleanor.lin@realtek.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>