staging: iio: tsl2x7x: rename driver for consistency with other IIO light drivers

This patch renames the tsl2x7x_core.c file to tsl2x7x.c so that the
naming convention is consistent with other IIO light drivers outside
of staging.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>