pinctrl: artpec6: Drop unused function and rename

The pincontrol callbacks used to be symmetric:
enable/disable.

However for a while we only have one function that
set the muxing, no disabling.

Drop the unused disable function, and rename the
*enable() function to *set().

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>