mfd: tc3589x: Improve function-level documentation

Use the correct function name in one case and adjust a variable name to
that of the corresponding function parameter in another case.

Issue detected using Coccinelle (http://coccinelle.lip6.fr/

)

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>