net: usbnet: Disable flow control on network transmit queue

For ipa bridge use case, data packets come directly from
ipa bridge instead of usual network stack. Hence there is
no need to stop/start network transmit queue based on TX
skb queue length which represents data packets arriving
from ipa bridge. During high throughput, TX skb queue remains
full causing network tx queue to be stopped. Tx queue staying
full for longer period of time, causing netdev watchdog followed
by deletion of client conntrack IP which results into data stall.

CRs-Fixed: 884128
Change-Id: Ia75eec549ca3c377a53fea10affeb1ed371c16ce
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>