[TG3]: Add tw32_wait_f() for some sensitive registers

The tw32_f() function (register write with immediate read flush) can
hang when used on some registers to switch clock frequencies and
power. A new tw32_wait_f() is added for such registers with the
delay before the read and after the read.

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