usb: host: remove unnecessary condition check

dma_pool_destroy() can handle NULL pointer correctly, so there is
no need to check NULL pointer before calling dma_pool_destroy().

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>