regulator: ltc3676: Simplify .readable_reg and .writable_reg callbacks

Use case range for continuous range to make the code shorter.
The .readable_reg and .writable_reg implementation are exactly the same,
so use a common ltc3676_readable_writeable_reg function instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Mark Brown <broonie@kernel.org>