mwl8k: dma header manipulations can't fail

Adding and removing the DMA header that the mwl8k hardware requires
on tx and provides on rx can never fail, since we are guaranteed to
have enough headroom on the tx path to expand the packet, and we only
ever shrink the packet on the rx path.  (And on both paths we are
guaranteed to be the only user of the skb we are handling.)

This allows removing all of the skb clone handling in the tx and tx
reclaim paths, and eliminates error checks in both the tx and rx
paths, simplifying the code a bit more.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>