wifi: rtl8xxxu: Allow setting rts threshold to -1

The default setting in hostapd.conf for rts threshold is -1, which means
disabled. Allow to set it.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230428150833.218605-6-martin.kaistra@linutronix.de