usb: host: Remove remaining pci_pool in comments

This replaces remaining occurences of pci_pool by dma_pool, as
this is the new API that could be used for that purpose.

Signed-off-by: Romain Perier <romain.perier@collabora.com>
Reviewed-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>