iio: iio-trig-periodic-rtc: Free trigger resource correctly

This is based on upstream commit 10e840dfb0b7, which did not touch the
iio-trig-periodic-rtc driver because it has been removed upstream.

The following explanation comes from that commit:

    These stand-alone trigger drivers were using iio_trigger_put()
    where they should have been using iio_trigger_free().  The
    iio_trigger_put() adds a module_put which is bad since they
    never did a module_get.

    In the sysfs driver, module_get/put's are used as triggers are
    added & removed. This extra module_put() occurs on an error path
    in the probe routine (probably rare).

    In the bfin-timer & interrupt trigger drivers, the module resources
    are not explicitly managed, so it's doing a put on something that
    was never get'd.  It occurs on the probe error path and on the
    remove path (not so rare).

    Tested with the sysfs trigger driver.
    The bfin & interrupt drivers were build tested & inspected only.

This was build tested only.

Cc: Alison Schofield <amsfield22@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>