smsc75xx: replace 0xffff with PHY_INT_SRC_CLEAR_ALL

This patch defines PHY_INT_SRC_CLEAR_ALL to replace the value 0xffff
in order to be more self-documenting.

This patch should make no functional change, it is purely cosmetic.

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