gpio: gpiolib: fix a doc warning

gpio: gpiolib: fix a doc warning

Use a different markup for the ERR_PTR, as %FOO doesn't work
if there are parenthesis. So, use, instead:

	``ERR_PTR(-EINVAL)``

This fixes the following warning:

	./drivers/gpio/gpiolib.c:139: WARNING: Inline literal start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/51197e3568f073e22c280f0584bfa20b44436708.1584456635.git.mchehab+huawei@kernel.org
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>