wifi: rtl8xxxu: Name some bits used in burst init

Use descriptive names instead of magic numbers.

Suggested-by: Ping-Ke Shih <pkshih@realtek.com>
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/e7d05bd9-e096-8361-f1b4-3c8b8599a7eb@gmail.com