USB: Allow transfer_buffer with transfer_dma

Some host controller drivers may need a PIO fallback when a DMA channel
is temporarily unavailable.  This patch provides an address that such
drivers can use for PIO in those cases, and nulls that field out when
no such address is available (highmem) which should help usbmon.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>