staging: iio: light: isl29018: use regmap for register access

Using regmap for accessing register through i2c bus. This will
remove the code for caching registers, read-modify-write logics.
Also it will provide the debugfs feature to dump register
through regmap debugfs.

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