soc: qcom: glink: Update header to clarify the proper usage

The glink_open function header was not clear enough about its usage.

Update header to state the proper way of invocation of the function.

Change-Id: Ic67745cab72de3bed403063174a1cb8aaa8991f9
Signed-off-by: Atish Kumar Patra <apatra@codeaurora.org>