hisax: Fix unchecked alloc_skb() return.

Jesper Juhl noticed that l2_pull_iqueue() does not
check to see if alloc_skb() fails.

Fix this by first trying to reallocate the headroom
if necessary, rather than later after we've made hard
to undo state changes.

Reported-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>