net: rmnet_data: Add default value for tx_queue_len

The default value of tx_queue_len was 0 which was causing packet
drops because of queueing in tc.
This patch sets a fixed value for the tx_queue_len

CRs-Fixed: 609873
Change-Id: I51739e63223b563f7cf8838d88908db7dcc9bf3e
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>