USB: build fix

this config:

http://redhat.com/~mingo/misc/config-Wed_Apr_30_15_12_48_CEST_2008.bad

fails to build due to an #error. Turn that into a #warning instead
to not break randconfig builds unnecessarily.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>