ieee802154: mcr20a: simplify a bit 'mcr20a_handle_rx_read_buf_complete()'

Use a 'skb_put_data()' variant instead of rewritting it.
The __skb_put_data variant is safe here. It is obvious that the skb can
not overflow. It has just been allocated a few lines above with the same
'len'.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Xue Liu <liuxuenetmail@gmail.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>