usb: common: usb-conn-gpio: Print error on failure to get VBUS

The exact error that happened trying to get the VBUS supply can be
useful to troubleshoot what's going on.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200806160248.3936771-2-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>