[ARM] pxa: fix pxa27x_udc default pullup GPIO

Currently, pxa27x_udc tries to use GPIO 0 as D+ pullup if not
explicitly configured. Default to an invalid GPIO (-1) instead.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>