spmi: pmic-arb: Return an error code if sanity check fails

If the test 'if (channel > 5)' is true, then we will return 'err' which
is known to be 0 at this point.
Return -EINVAL instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>