regulator: tps65090: Fix tps65090 crash on Tegra Dalmore

The patch (60e91b5 regulator: tps65090: Allow setting the overcurrent
wait time) introduced a crash on Tegra Dalmore.  On Dalmore the device
tree doesn't have an entry for all of the FETs so it leaves tps_pdata
NULL in some cases.  Add a check for NULL like the rest of the code
does.

Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>