USB: remove warn() macro from usb.h

USB should not be having it's own printk macros, so remove warn() and
use the system-wide standard of dev_warn() wherever possible.  In the
few places that will not work out, use a basic printk().

Now that all in-tree users are gone, remove the macro.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>