thermal: rockchip: improve conversion error messages

These error messages don't give much information about what went wrong.
It would be nice, for one, to see what invalid temperature was being
requested when conversion fails. It's also good to return an error when
we can't handle a conversion properly.

While we're at it, fix the grammar too.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>