tg3: Fix NVRAM size detection for the STM45PE20 pinstrap on 5762 devices

The STM45PE20 pinstrap on 5762 devices supports multiple sizes. So treat
it just like the ST45_USPT and the size will be read from 0xf0 via
tg3_get_nvram_size().

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>