dfc: Cache grants before flow is added

If a grant indication is received before the corresponding flow
is added, the grant information needs to be cached for use later
instead of ignored which could lead to data stall.

This change also adjust flow control ftrace prints.

Change-Id: I78ab6a6f0a046e92eac3d477cab6ec33b1109171
Acked-by: Weiyi Chen <weiyic@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>