net/macb: fix truncate warnings

When building macb on x86_64 the following warnings show up:
  drivers/net/ethernet/cadence/macb.c: In function macb_interrupt:
  drivers/net/ethernet/cadence/macb.c:556:4: warning: large integer implicitly truncated to unsigned type [-Woverflow]
  drivers/net/ethernet/cadence/macb.c: In function macb_reset_hw:
  drivers/net/ethernet/cadence/macb.c:792:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]
  drivers/net/ethernet/cadence/macb.c:793:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]
  drivers/net/ethernet/cadence/macb.c:796:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]

Use -1 insted of ~0UL, as done in other places in the driver,
to silence these warnings.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>