power: supply: ab8500_btemp: Compress return logic into one line.

Simplify return logic to avoid unnecessary variable assignments.
These issues were detected using Coccinelle and the following semantic patch:

@@
local idexpression ret;
expression e;
@@

-ret =
+return
     e;
-return ret;

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>