hal-api: power: Use usage_count to support multiple call

hal-api: power: Use usage_count to support multiple call

The user of hal-api-power such as PASS daemon can execute
the hal_power_get_backend and hal_power_put_backend at multiple times.

Until now, hal-api-power doesn't support the multiple function call
of hal_power_get_backend/put_backend. It cause that return error value
when calling hal_power_put_backend from user of hal-api-power.

In order to fix this issue, use usage_count to put the hal backend
when usage_count is zero.

Change-Id: Idc1aef66e09e335cc158ace4c359673ff45792b8
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>