tg3: Ensure boot code has completed initialization before accessing hardware

After resetting the device, the driver waits for a signature to be
updated to know that firmware has completed initialization. However, the
call to tg3_poll_fw() is being done too late and we're writing to the
GRC_MODE register before it has completely initialized, causing
contention with firmware.  This logic has existed since day one but is
causing PCIE link to go down randomly at startup on one platform once
every few hundred reboots.

Move the tg3_poll_fw() up to before we write to the GRC_MODE register
after reset.

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>