wifi: rtl8xxxu: Fix off by one initial RTS rate

rtl8xxxu_set_basic_rates() sets the wrong initial RTS rate. It sets the
next higher rate than the one it should set, e.g. 36M instead of 24M.

The while loop was supposed to find the index of the most significant
bit which is 1, but it was copied incorrectly from the vendor driver.
Use __fls() instead.

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://msgid.link/761e6836-6cd6-4930-91b6-0446834655c5@gmail.com