mfd: axp20x: Correct a typo in axp20x_device_remove documentation

The documentation of axp20x_device_remove() have a typo and use
axp20x_device_probe() as name. This patch fix this typo.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>