ftgmac100: Rework MAC reset and init

The HW requires a full MAC reset when changing the speed.

Additionally the Aspeed documentation spells out that the
MAC needs to be reset twice with a 10us interval.

We thus move the speed setting and top level reset code
into a new ftgmac100_reset_and_config_mac() function which
handles both. Move the ring pointers initialization there
too in order to reflect the HW change.

Also reduce the timeout for the MAC reset as it shouldn't
take more than 300 clock cycles according to the doc.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>