dfc: fix null pointer access

Fix a null pointer dereference issue when data packets trigger the
queuing of powersave work before the powersave workqueue is initialized.

Change-Id: Ia3515a7aaa47cb41568c39462bca73ceae11ea9c
Acked-by: Weiyi Chen <weiyic@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>