drivers: ethernet: eth_stm32 add tx async mode

Enable TX transmission asynchronous mode to enhance performance

Tested on the nucleo_h743zi board using samples/net/zperf

west build -b nucleo_h743zi/stm32h743xx samples/net/zperf/ --pristine
-- -DCONFIG_NET_CONFIG_MY_IPV4_ADDR=\"192.168.1.199\"

upload cmd:
```
//linux
iperf -s -l 1K -p 5001
iperf -s -l 1K -p 5002 -u

//shell
zperf tcp upload 192.168.1.109 5001 10 1K
zperf udp upload 192.168.1.109 5002 10 1K 300M
```

download cmd:
```
//shell
zperf tcp download 5001 192.168.1.199
zperf ucp download 5002 192.168.1.199

//linux
iperf -l 1K -c 192.168.1.199 -p 5001
iperf -l 1K -c 192.168.1.199 -p 5002 -u
```

before:
udp upload:   73.5Mbps/s
tcp upload:   71.3Mbps/s
udp download: 93.9Mbps/s
tcp download: 70.5Mbps/s

after:
udp upload:   92.2Mbps/s
tcp upload:   78.1Mbps/s
udp download: 93.7Mbps/s
tcp download: 75.2Mbps/s

Signed-off-by: WenBin Zhang <freey7955@gmail.com>