net: ipa: set DMA length in gsi_trans_cmd_add()

When a command gets added to a transaction for the AP->command
channel we set the DMA address of its scatterlist entry, but not
its DMA length.  Fix this bug.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>