cpufreq: mediatek-hw: don't use error path on NULL fdvfs

cpufreq: mediatek-hw: don't use error path on NULL fdvfs

The IS_ERR_OR_NULL check for priv->fdvfs is inappropriate, and should be
an IS_ERR check instead, as a NULL value here would propagate it to
PTR_ERR.

In practice, there is no problem here, as devm_of_iomap cannot return
NULL in any circumstance. However, it causes a Smatch static checker
warning.

Fix the warning by changing the check from IS_ERR_OR_NULL to IS_ERR.

Fixes: 32e0d669f3ac ("cpufreq: mediatek-hw: Add support for MT8196")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-pm/aKQubSEXH1TXQpnR@stanley.mountain/
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>