sky2: Receive Overflows not counted

The sky2 driver doesn't count the Receive Overflows because the MAC
interrupt for this event is not set in the MAC's interrupt mask.
The MAC's interrupt mask is set only for Transmit FIFO Underruns.

Fix: The correct setting should be (GM_IS_TX_FF_UR | GM_IS_RX_FF_OR)
Otherwise the Receive Overflow event will not generate any interrupt.
The  Receive Overflow interrupt is handled correctly

Signed-off-by: Mirko Lindner <mlindner@marvell.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>