pinmux: generic: Use ENOENT instead of ENOSYS

ENOSYS should only be used when a subsystem is completely absent.
Convert its use in pinctrl-generic to ENOENT, which better reflects the
error condition (that a function/group/pin is missing).

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>