tsnep: Fix NAPI polling with budget 0

[ Upstream commit 46589db3 ]

According to the NAPI documentation networking/napi.rst, Rx specific
APIs like page pool and XDP cannot be used at all when budget is 0.
skb Tx processing should happen regardless of the budget.

Stop NAPI polling after Tx processing and skip Rx processing if budget
is 0.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>