net: rmnet_data: Use hrtimer for UL aggregation timer

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: 2157214
Change-Id: I29cbbee417e84b101ed34c0d29c2731bd52c3ec5
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>