power: supply: test-power: revise parameter printing to use sprintf

Simplify code by using sprintf instead of strcpy+strcat+strlen.

Signed-off-by: Harley A.W. Lorenzo <hl1998@protonmail.com>
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>