gpiolib: Drop cargo-culted comment

This comment about the gpio_lock is just completely confusing and
misleading. This refers to a gpio_desc that would in 2008 be used
to hold the list of gpio_chips, but nowadays gpio_desc refers to
descriptors of individual GPIO lines and this comment is completely
unparseable. Delete it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>