power: supply: lp8788: prevent out of bounds array access

val might become 7 in which case stime[7] (array of length 7) would be
accessed during the scnprintf call later and that will cause issues.
Obviously, string concatenation is not intended here so just a comma needs
to be added to fix the issue.

Fixes: 98a27664 ("power_supply: Add new lp8788 charger driver")
Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>