msm: mhi_dev: Support async write in UCI for size greater than 8k

In the current implementation of MHI UCI layer, write operation from
device to host fails if the size requested by client is more than 8k.
Removing this condition in this change as MHI supports buffer size of
upto 64k in async path. Continuing to fail write in sync case, as MHI
layer uses pre allocated buffers of size 8k. Also, added debug logs to
check if buffer size of Diag is more than 16k. With this change async
writes of size greater than 8k are supported in UCI layer.

Change-Id: I084da6a49a00095e806872f591365eabb9edc1de
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>