gpio: stp-xway: improve module clock error handling

gpio: stp-xway: improve module clock error handling

Three module clock error handling improvements:
- use devm_clk_get() so the clock instance can be freed if
  devm_gpiochip_add_data() fails later on
- switch to clk_prepare_enable() so the driver is ready whenever the
  lantiq target switches to the common clock framework
- disable the clock again (using clk_disable_unprepare()) if
  devm_gpiochip_add_data()

All of these are virtually no-ops with the current lantiq target.
However, these will be relevant if we switch to the common clock
framework.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20190702223248.31934-3-martin.blumenstingl@googlemail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>