net/macb: suppress compiler warnings

This patch fixes the following warnings:
drivers/net/ethernet/cadence/macb.c: In function ‘macb_handle_link_change’:
drivers/net/ethernet/cadence/macb.c:266: warning: comparison between signed and unsigned
drivers/net/ethernet/cadence/macb.c:267: warning: comparison between signed and unsigned
drivers/net/ethernet/cadence/macb.c:291: warning: comparison between signed and unsigned
drivers/net/ethernet/cadence/macb.c: In function ‘gem_update_stats’:
drivers/net/ethernet/cadence/macb.c:1908: warning: comparison between signed and unsigned
drivers/net/ethernet/cadence/macb.c: In function ‘gem_get_ethtool_strings’:
drivers/net/ethernet/cadence/macb.c:1988: warning: comparison between signed and unsigned

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>