fix USB devices with multiple configurations

This patch fixes bugs in usbdcore*.c related to the use of devices
with multiple configurations.

The original code made mistakes about the meaning of configuration value and
configuration index, and the resulting off-by-one errors resulted in:

* SET_CONFIGURATION always selected the first configuration, no matter what
  wValue is being passed.
* GET_DESCRIPTOR/CONFIGURATION always returned the descriptor for the first
  configuration (index 0).

Signed-off-by: Harald Welte <laforge@openmoko.org>
Acked-by: Markus Klotzbuecher <mk@denx.de>