net: rmnet_data: Go back to worker thread for UL aggregation

A crash was seen when running the hrtimer solution, so revert the
change till it is resoloved. The crash occurs due to interrupts
being enabled by a later function in the call stack within
interrupt context.

[ffffff8cc22b157c] __local_bh_enable_ip+0xa8/0x114
[ffffff8cc33495d0] _raw_spin_unlock_bh+0x30/0x3c
[ffffff8cc2e57ed8] ipa3_send+0x2a0/0xaa0
[ffffff8cc2e5c544] ipa3_tx_dp+0x59c/0x990
[ffffff8cc2ea7ef0] ipa3_wwan_xmit+0x204/0x338
[ffffff8cc30f0160] dev_hard_start_xmit+0xc4/0x29c
[ffffff8cc311d0f4] sch_direct_xmit+0x100/0x1cc
[ffffff8cc30f0890] __dev_queue_xmit+0x1f4/0x704
[ffffff8cc30f0dc8] dev_queue_xmit+0x28/0x34
[ffffff8cc333285c] rmnet_map_flush_packet_queue+0xc0/0x218
[ffffff8cc2344f9c] __hrtimer_run_queues+0x158/0x36c
[ffffff8cc2345f38] hrtimer_interrupt+0xb0/0x1f4

CRs-fixed: 2147503
Change-Id: I7b3617577cbf5e166380d4361b73e2f57a4bd042
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>