r8152: avoid start_xmit to schedule napi when napi is disabled

[ Upstream commit de9bf29dd6e4a8a874cb92f8901aed50a9d0b1d3 ]

Stop the tx when the napi is disabled to prevent napi_schedule() is
called.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>