staging: iio: light: of: Fix vendor prefix of isl29018/isl29028

ISL29018/ISL29028 is from Intersil Corporation and making the
vendor prefix for this part as "isil" for OF compatibity.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>