staging: rtl8192u: check dev_alloc_skb() return value in fw_download_code()

This patch adds a check whether skb allocation, in fw_download_code(), was
successful.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>