atmel_usba_udc: Kill GPIO_PIN_NONE

GPIO_PIN_NONE should no longer be used. Replace it with a simple
test against negative values.

This is a transitional patch, waiting for gpio_is_valid() to be
merged at which point the tests should be revisited.

Signed-off-by: Stelian Pop <stelian@popies.net>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>