power: twl4030: fix a missing check of return value

If twl4030_bci_read() fails, the read data in "s" is incorrect,
which is however used in the following execution. The fix checks
the return value of twl4030_bci_read() and returns an error code
upstream upon the failure of twl4030_bci_read().

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>