jz4740-battery: Fix signedness bug

wait_for_completion_interruptible_timeout() may return negative value.
In this case, checking if (t > 0)  will return true if t is unsigned.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>