net: rmnet_data: Stop adding pad bytes for MAPv3 uplink packets

Hardware does not require pad bytes in egress packets when uplink
aggregation is not enabled.

CRs-Fixed: 1002396
Change-Id: I86459b7bc18da16b66f6c701ac324f28be8848fa
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>