generic-adc-battery: Fix checking return value of request_any_context_irq

On failure, request_any_context_irq() returns a negative value. On
success, it returns either IRQC_IS_HARDIRQ or IRQC_IS_NESTED.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>