mhi: dev: uci: Avoid resubmission of same buffer to MHI

mhi_uci_read() drops and reacquires spin_lock when performing
copy_to_user(). However, there is a possibility of two user
threads getting hold of same uci_buf when the lock is released.
This can result in submission of same uci_buf to MHI which can
later result in list corruption when mhi_dl_xfer_cb() tries to
put same uci_buf to pending list twice.

Change-Id: Iabcd6f11acc1a0852811f9c78d7d7f073aa5c724
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>