clk: qcom: wait for backend response forever in virtio clock driver

Since guest virtual machine may be sheduled out by host for an uncertain
time, we change the waiting for backend response from 200ms timeout to
no timeout.

Change-Id: I75b4eb2435a32ed6b322046cc49ea9bd9706fe6b
Signed-off-by: Zhiqiang Tu <ztu@codeaurora.org>