PM / devfreq: mtk-cci: Fix potential error pointer dereference in probe()

The drv->sram_reg pointer could be set to ERR_PTR(-EPROBE_DEFER) which
would lead to a error pointer dereference.  Use IS_ERR_OR_NULL() to check
that the pointer is valid.

Fixes: e09bd575 ("PM / devfreq: mtk-cci: Handle sram regulator probe deferral")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://patchwork.kernel.org/project/linux-pm/patch/aJTNHz8kk8s6Q2os@stanley.mountain/