mhi: core: prepare all the descriptors before ringing hardware doorbell

While mhi host queues new buffers to transfer ring during
start, process completion transfer thread will queue a
buffer to same channel after receiving a completion
event. To avoid this scenario, prepare all descriptors
before ringing the hardware doorbell so there will not be
any completion packets to process.

CRs-Fixed: 2236500
Change-Id: I91f065932ef46dc3520fd1a8ad634e2b65badff1
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>