power: pm8921-bms: fix 0% soc reported when started with a charger

When the device is started with a charger inserted, it reports 0% soc.
This is because a variable "soc_at_cv" is not initialized correctly in
the probe function.

Fix this by initializing it appropriately to -EINVAL.

Change-Id: I106f36e204c5cc4f16a8f52b9fa7a2a012282e35
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>