thermal: kirkwood: Fix valid check for thermal register

The correct value is obtain by first shifting the register by the offset,
later applying the valid mask and finally invert the result.
This check was lacking an extra parenthesis to be strictly correct.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>