USB: fix signedness issue in drivers/usb/gadget/ether.c

another gcc 4.1 signdness warning:

drivers/usb/gadget/ether.c:2028: warning: comparison of unsigned expression < 0 is always false

length is assigned the value of usb_ep_queue() which returns an int.
Directly after this it is checked for < 0, which can never be true.  Making
length an int makes the error check work again.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>