rtlwifi: fix potential NULL pointer dereference

[ Upstream commit 60209d482b97743915883d293c8b85226d230c19 ]

In case dev_alloc_skb fails, the fix safely returns to avoid
potential NULL pointer dereference.

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