greybus: es2: don't protest when getting -ENOENT/ESHUTDOWN USB errors

-ENOENT or -ESHUTDOWN happens when the urbs are being killed from
->disconnect() callback.  Don't complain to userspace about this, as the
user will see this on es2 module removal.

Tested by removing the gb-es2.ko module on both EVT1.5 and qemu with
gbsim. The driver doesn't throw anymore errors like:
"urb cport in error -2 (dropped)".

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: David Lin <dtwlin@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>