rtc: m41t80: m41t80_sqw_set_rate should return 0 on success

commit de6042d2fa8afe22b76e3c68fd6e9584c9415a3b upstream.

Previously it was returning -EINVAL upon success.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>