rmnet_shs: Make prio switch RR

Before this change NET_RX priority switching was not round
robin within a shs wq tick.

This meant if many heavy flows were created in the same wq tick
then they would be placed on the same core. This can cause packet
loss if such flows are too heavy for a single gold core.

This change will cause prio flows to select the core with the least
number of active flows for their switch. This will cause a RR effect
when they come within the same wq tick.

CRs-Fixed: 2448720
Change-Id: I4b4edea746270a880e3a22a09730d15d16a392ab
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>