rtw88: Don't set RX_FLAG_DECRYPTED if packet has no encryption

The value of GET_RX_DESC_SWDEC() indicates that if this RX
packet requires software decryption or not. And software
decryption is required when the packet was encrypted and the
hardware failed to decrypt it.

So, GET_RX_DESC_SWDEC() is negative does not mean that this
packet is decrypted, it might just have no encryption at all.
To actually see if the packet is decrypted, driver needs to
further check if the hardware has successfully decrypted it,
with a specific type of encryption algorithm.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>