xhci: Handle zero-length isochronous packets.

xhci: Handle zero-length isochronous packets.

For a long time, the xHCI driver has had this note:
	/* FIXME: Ignoring zero-length packets, can those happen? */

It turns out that, yes, there are drivers that need to queue zero-length
transfers for isochronous OUT transfers.  Without this patch, users will
see kernel hang messages when a driver attempts to enqueue an isochronous
URB with a zero length transfer (because count_isoc_trbs_needed will return
zero for that TD, xhci_td->last_trb will never be set, and updating the
dequeue pointer will cause an infinite loop).

Matěj ran into this issue when using an NI Audio4DJ USB soundcard
with the snd-usb-caiaq driver.  See
	https://bugzilla.kernel.org/show_bug.cgi?id=40702

Fix count_isoc_trbs_needed() to return 1 for zero-length transfers (thanks
Alan on the math help).  Update the various TRB field calculations to deal
with zero-length transfers.  We're still transferring one packet with a
zero-length data payload, so the total_packet_count should be 1. The
Transfer Burst Count (TBC) and Transfer Last Burst Packet Count (TLBPC)
fields should be set to zero.

This patch should be backported to kernels as old as 2.6.36.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Matěj Laitl <matej@laitl.cz>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: stable@kernel.org