gpio: menz127: Drop lock field from struct men_z127_gpio

Current code uses a uninitialized spin lock.
bgpio_init() already initialized a spin lock, so let's switch to use
&gc->bgpio_lock instead and remove the lock from struct men_z127_gpio.

Fixes: f436bc27 "gpio: add driver for MEN 16Z127 GPIO controller"
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>