drivers: qcom: ipc_router_hsic_xprt: Clear the subsystem restart flag

When the peripheral resets and then boots up, the work item to read the
data from the underlying transport exits since the link is in reset state.
This in turn causes the IPC Router initialization handshake protocol to
stop prematurely.

Clear the subsystem restart flag once the link is up.

CRs-Fixed: 676911
Change-Id: Iea24fa172ba9817849b35b79822c7a61833695a1
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>