regulator: tps65090: Fix gpio initialization

The config is used for multiple regulators within a for loop. The config
field is not cleared before it is used for the next item. To avoid any
issues this patch adds a proper initialization for the config->ena_gpio
field in case no gpio is available.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>