tg3: Fix 5761 NVRAM sizes

The 5761 NVRAM sizes assigned to the nvram_size member are half as big
as they should be.  This patch corrects the NVRAM sizes and replaces
the hardcoded constants with preprocessor constants for readability.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>