gpio: stp-xway: simplify error handling in xway_stp_probe()

gpio: stp-xway: simplify error handling in xway_stp_probe()

Return early if devm_gpiochip_add_data() returns an error instead of
having two consecutive "if (!ret) ..." statements.

Also make xway_stp_hw_init() return void because it unconditionally
returns 0. While here also update the kerneldoc comment for
xway_stp_hw_init().

These changes makes the error handling within the driver consistent.
No functional changes intended.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20190702223248.31934-2-martin.blumenstingl@googlemail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>