[FORCEDETH] Fix xmit_lock/netif_tx_lock after merge

There has been an update to the forcedeth driver that added a few new
uses of xmit_lock which is no longer meant to be used directly.  This
patch replaces them with netif_tx_lock_bh.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>