rapidio/tsi721_dma: fix failure to obtain transaction descriptor

rapidio/tsi721_dma: fix failure to obtain transaction descriptor

This is a bug fix for the situation when function tsi721_desc_get() fails
to obtain a free transaction descriptor.

The bug usually results in a memory access crash dump when data transfer
scatter-gather list has more entries than size of hardware buffer
descriptors ring.  This fix ensures that error is properly returned to a
caller instead of an invalid entry.

This patch is applicable to kernel versions starting from v3.5.

Signed-off-by: Alexandre Bounine <[email protected]>
Cc: Matt Porter <[email protected]>
Cc: Andre van Herk <[email protected]>
Cc: Stef van Os <[email protected]>
Cc: Vinod Koul <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: <[email protected]>	[3.5+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>