gianfar: Restore link state settings after MAC reset

There are some MAC registers that need to be kept in sync
with the link state parameters, see adjust_link().
However, after a MAC soft reset default values for
these registers are assumed.  In some cases (excepting
if down/ if up for example) adjust_link() does not see
that these values were reset to default because the
priv->old* link parameters were left unchanged.
So, reset the priv->old* link params as well during a
MAC reset to let adjust_link() restore the MAC link
settings to the actual link state values.

Fixes following case, for example:
Setting link to 100M, changing MTU (implies MAC reset),
link state remains unchanged to 100M but MAC registers
were reset to default (1G) breaking the connectivity w/
the PHY.  Closing and re-opening the interface would
restore the MAC link parameters to the correct values.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>