net: rmnet_data: modify the packet logging

skb->tail and skb->end are defined as character pointers in 32 bit
environments and as integers in other environments. Fix compilation
issues seen in the packet logging functions as a result of the data
type mismtach in 64 bit environments.

CRs-fixed: 665364
Change-Id: Ie70e01ce0678947d9c8cd924fe99b89ce319d4e5
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>