i2c-msm-v2: use a single sg-list per user request and per chan

Rather than allocating a scatter-gather list per user's
transfer buffer, allocate a single scatter gather list per
channel(rx or tx) per user's transfer request and queue all
the respective transfer buffers in it.

Change-Id: I9f486fad71777b7419f6d49378eb87e998f55ec8
Signed-off-by: Ankit Gupta <ankgupta@codeaurora.org>