regulator: fan53555: Check pdata->slew_rate setting

Current code does not really avoid array access out of bounds,
fix it by add checking for pdata->slew_rate.
If pdata->slew_rate is too big, it's a bug in pdata that needs fix.

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