mhi: core: start UL channel before DL channel

For some MHI clients, initial handshake starts from device.
Soon as host starts device to host (DL) channel, device sends
initial 'hello' packet to host. Then client immediately try to
respond back to device but fails since uplink (UL) channel is
not yet started. To avoid this race condition, always prepare
the UL channel first. So, channel is ready to respond to any
packets from device.

CRs-Fixed: 2229675
Change-Id: If0f196a93c4912d7109176ee814deb39c1ac090a
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>