power_supply: ipaq_micro_battery: Fix leaking workqueue

Driver allocates singlethread workqueue in probe but it is not destroyed
during removal.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Fixes: 00a588f9 ("power: add driver for battery reading on iPaq h3xxx")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>