mhi: uci: Fix a bug related to split transfer packets

When transfer buffer is larger than available space, uci driver will
split the transfer into multiple transactions. Driver incorrectly
calculated the transfer length and caused infinite transfer.
Simplify uci write method to avoid such bugs.

CRs-Fixed: 2083693
Change-Id: Ic7169cefda6a4637511ecfa3ce5ddde6f3d55f8c
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>