slimbus: qcom-ngd: Fix kfree() of static memory on setting driver_override

The driver_override field from platform driver should not be initialized
from static memory (string literal) because the core later kfree() it,
for example when driver_override is set via sysfs.

Use dedicated helper to set driver_override properly.

Fixes: 917809e2

 ("slimbus: ngd: Add qcom SLIMBus NGD driver")
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419113435.246203-11-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>