Fix desc_get to alloc a descriptor from the cache if the ones in the
list are waiting for the ack.  Also, memzero the descriptor when
allocated from the list to ensure all fields are cleared.

Acked-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
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