usb: dwc3: omap: Fix devm_regulator_get_optional() error handling

usb: dwc3: omap: Fix devm_regulator_get_optional() error handling

Commit 533561a8aad5 ("usb: dwc3: omap: Use
devm_regulator_get_optional()") assumed NULL was returned, but
devm_regulator_get_optional() returns -ENODEV rather than NULL like
other *_get_optional() functions.

Fixes: 533561a8aad5 ("usb: dwc3: omap: Use devm_regulator_get_optional()")
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250109170748.3852439-1-robh@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>