USB: isp1362: remove unused _WARN_ON() calls

Like _BUG_ON(), _WARN_ON() wasn't ever being used, so just delete it, as
obviously things are working properly now (if not, we have bigger
problems...)

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>