net: rmnet_data: Optimize the UL aggregation skip logic

Optimize for the IPv6 normal ping case instead of the
fragmentation scenario. Also, check for IPv6 explicitly
rather than directly disecting the packet.

CRs-Fixed: 2131879
Change-Id: Idee5c7d52972d06aee6ac6d71ebd5d1c3c875f9f
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>