staging: rtl8822be: fix missing null check on dev_alloc_skb return

dev_alloc_skb can potentially return NULL, so add a null check to
avoid a null pointer dereference on skb

Detected by CoverityScan, CID#1454558 ("Dereference on null return")

Fixes: 7e5b796c ("staging: r8822be: Add the driver code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>