net: macb: remove redundant assignment to variable work_done

Variable work_done is set to zero and this value is never read, instead
it is set to another value a few statements later.  Remove the redundant
assignment. Cleans up clang warning:

drivers/net/ethernet/cadence/macb_main.c:1221:2: warning: Value stored
to 'work_done' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>