wifi: rtw89: extract link part from core tx write function

Extract core_tx_write_link from core_tx_write to accept given links as
parameters. To make the follow-up changes of TX description more clear,
tweak SW functions to split things ahead.

(don't change logic at all)

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250505072440.45113-2-pkshih@realtek.com