thermal/drivers/hisi: Remove pointless irq field

The irq field in the data structure is pointless as the scope of its
usage is just to request the interrupt. It can be replaced by a local
variable.

Use the 'ret' variable to get the interrupt number.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>