pinctrl/pinctrl-u300: remove devm_kfree at driver unload

the memory allocated by devm_kzalloc is automatically
freed at the driver detach side, so no neeed of calling
devm_kfree

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>