wifi: rtlwifi: simplify TX command fill callbacks

Since 'rtlpriv->cfg->ops->fill_tx_cmddesc()' is always called
with 'firstseg' and 'lastseg' set to 1 (and the latter is
never actually used), all of the relevant chip-specific
routines may be simplified. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231011154442.52457-2-dmantipov@yandex.ru