wifi: rtw88: main: Reserve 8 bytes of extra TX headroom for SDIO cards

For SDIO host controllers with DMA support the TX buffer physical memory
address need to be aligned at an 8-byte boundary. Reserve 8 bytes of
extra TX headroom so we can align the data without re-allocating the
transmit buffer.

While here, also remove the TODO comment regarding extra headroom for
USB and SDIO. For SDIO the extra headroom is now handled and for USB it
was not needed so far.

Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230405200729.632435-6-martin.blumenstingl@googlemail.com