smsc95xx: fix function prototype of async callback

smsc95xx_async_cmd_callback doesn't currently match usb_complete_t, so
there's a cast to force the square peg into the round hole.  This patch
fixes this properly.

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