usb: core: Do not choose configuration if actconfig is valid

Consider a case where some specific entity like userspace or
libusb has already set some configuration before usb_probe_device
could get invoked. Then usb_choose_configuration sets the config
back to some default config which can lead to desired functions
not getting chosen.
To fix this, do not allow usb_choose_configuration to set any
config if udev->actconfig is valid.

Change-Id: If5166c0610a8c402929e52a4e11a77920dee0356
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>