net: lwip: ping: set net_try_count to 1

net: lwip: ping: set net_try_count to 1

The legacy network stack sets net_try_count to 1 at the beginning of the
net_loop() function. This is required for net_start_again() to work
properly. Therefore, set the variable accordingly in the do_ping()
function when NET_LWIP=y. This fixes an issue where a ping to an
unreachable destination would run twice on the same network device. For
example with qemu_arm64_lwip_defconfig:

 => dhcp
 DHCP client bound to address 10.0.2.15 (3 ms)
 => ping 10.0.0.1
 Using virtio-net#32 device
 ping failed; host 10.0.0.1 is not alive
 Using virtio-net#32 device
 ping failed; host 10.0.0.1 is not alive
 => QEMU: Terminated

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>