[TG3]: ethtool -d hangs PCIe systems

Resubmitting after recommendation to use GET_REG32_1() instead of
GET_REG32_LOOP(..., 1).  Retested.  Problem remains fixed.

Prevent tg3_get_regs() from reading reserved and undocumented registers
at RX_CPU_BASE and TX_CPU_BASE offsets which caused hostile behavior
on PCIe platforms.

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