pinctrl: single: fix freudian slip

commit 03e9f0ca
"pinctrl: clean up after enable refactoring"
renamed the vtable callback .enable to .set_mux. The
renaming was done manually, and one of the alterations
contained a freudian slip. I confess, I am human.

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