dsa_master_find_slave()'s time complexity and potential performance hit

[DENG Qingfang]

Since commit 7b9a2f4bac68 ("net: dsa: use ports list to find slave"),
dsa_master_find_slave() has been iterating over a linked list instead
of accessing arrays, making its time complexity O(n).
The said function is called frequently in DSA RX path, so it may cause
a performance hit, especially for switches that have many ports (20+)
such as RTL8380/8390/9300 (There is a downstream DSA driver for it,
see https://github.com/openwrt/openwrt/tree/openwrt-21.02/target/linux/realtek/files-5.4/drivers/net/dsa/rtl83xx).
I don't have one of those switches, so I can't test if the performance
impact is huge or not.