spi: release device claimed by bus_find_device_by_name

In success case the function bus_find_device_by_name calls
get_device. In our context put_device should be called to
decrease the device count usage.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>