power: supply: isp1704: remove redundant assignment to variable ret

The variable ret is being assigned with a value that is never
read and it is being updated later with a new value. The
assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>