pinctrl: plgpio: Call clk_disable_unprepare only if clk_prepare_enable is called

This driver allows clk_get() failure, and still work without it.
This patch adds !IS_ERR(plgpio->clk) checking in plgpio_request() error path
so we only call clk_disable_unprepare() if clk_prepare_enable() is called.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>