net: qualcomm: rmnet: Add support for UL aggregation

This patch brings in the implementation for the UL aggregation
for packets sent from rmnet. The delay argument in
schedule_delayed_work(struct delayed_work *dwork, unsigned long
delay) API is jiffies. The system tick for seems to be 100Hz, so
the minimum time resolution for the work to be scheduled is 10ms.

Switch to hrtimer to achieve 3 ms granularity with a current timer
of 1ms for the flush thread. A workqueue is immediately scheduled
in the same context after this timer expiry to do the tx work.

CRs-Fixed: 2156182
Change-Id: I6beb47d86b133b1db85d9a352eae8870e554eebb
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>