pch_gbe: Use PCH_GBE_PHY_REGS_LEN instead of 32

Avoid using magic numbers when we have perfectly good defines just lying
around.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Waskiewicz <peter.p.waskiewicz.jr@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: netdev@vger.kernel.org