drivers: usb: device: stm32: fix disconnect-gpios configuration.

For some reason, rather than testing for the presence of
disconnect-gpios in the devicetree, the STM32 USB Device driver was
relying on a hidden Kconfig flag to be set.

This patch removes the Kconfig option completely and simply tests for
the DT property - if it's set, you obviously know what you're doing and
obviously need the pull-up GPIO behaviour.

Signed-off-by: Chris Collins <chris@realsimgear.com>