regulator: tps65090: Make FETs more reliable by adding retries

An issue was discovered with tps65090 where sometimes the FETs
wouldn't actually turn on when requested (they would report
overcurrent).  The most problematic FET was the one used for the LCD
backlight on the Samsung ARM Chromebook (FET1).  Problems were
especially prevalent when the device was plugged in to AC power (when
the backlight voltage was higher).

Mitigate the problem by adding retries on the enables of the FETs,
which works around the problem fairly effectively.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michael Spang <spang@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mark Brown <broonie@linaro.org>