via-rhine: Fix bugs in NAPI support.

1) rhine_tx() should use dev_kfree_skb() not dev_kfree_skb_irq()

2) rhine_slow_event_task's NAPI triggering logic is racey, it
   should just hit the interrupt mask register.  This is the
   same as commit 7dbb4918


   ("r8169: avoid NAPI scheduling delay.") made to fix the same
   problem in the r8169 driver.  From Francois Romieu.

Reported-by: Jamie Gloudon <jamie.gloudon@gmail.com>
Tested-by: Jamie Gloudon <jamie.gloudon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>