power_supply: Fix race in power_supply_uevent

Commit 54d29ad3 (Power Supply: fix race
in device_create) introduced a race in power_supply_uevent. Previously it
checked that power_supply is available by checking for dev->driver_data.
But now dev->driver_data is set before power_supply->dev is initialised.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>