soc: qcom: dfc: Change workqueue type to single thread

The max_active flag is not sufficient to guarantee that only one
instance of the work for dfc_do_burst_flow_control is running
across all cores. Change the allocation API to guarantee this behavior.

Additionally, remove all references to disabling bottom half processing
as SOFTIRQ NET_TX cannot invoke this thread as the transmit path no
longer calls the rtnl lock.

CRs-Fixed: 2303876
Change-Id: I5ecd7edc4c7d75b6b8df98a42d011b19e1fa75c0
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>