qtnfmac: Simplify code in _attach functions

There is no need to re-implement 'netdev_alloc_skb_ip_align()' here.
Keep the code simple.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200407193233.9439-1-christophe.jaillet@wanadoo.fr