wifi: rtl8xxxu: Introduce rtl8xxxu_update_ra_report

The ra_report struct is used for reporting the TX rate via
sta_statistics. The code which fills it out is duplicated in two
places, and the RTL8188EU will need it in a third place. Move this
code into a new function rtl8xxxu_update_ra_report.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/0777ad35-fe03-473c-2e02-e3390bef5dd0@gmail.com