rtlwifi: fix potential NULL pointer dereference

commit 60209d48 upstream.

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: Paul Gortmaker <paul.gortmaker@windriver.com>