mv643xx_eth: delete unused and uninteresting interrupt source mask bits

Delete a couple of unused and uninteresting interrupt source mask bits:
- The receive resource underrun interrupt sources are uninteresting
  because if we are in out-of-memory mode, we are already dealing with
  the issue, and we don't need the hardware to remind us again that we
  are out of memory.
- The LINK and PHY interrupt sources can be coalesced into one define,
  since we always use them together.
- The transmit resource underrun interrupt source can be disabled since
  we never activate the head descriptor of a paged skb until the
  fragments are all activated, so transmit underrun during a packet
  should never happen.
- The INT_EXT_TX_0 define is never used.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>