smsc95xx: fix async register writes on big endian platforms

This patch fixes a missing endian conversion which results in the
interface failing to come up on BE platforms.

It also removes an unnecessary pointer dereference from this
function.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>