net: qcom/emac: add ethtool support for setting ring parameters

Implement the set_ringparam method, which allows the user to specify
the size of the TX and RX descriptor rings.  The values are constrained
to the limits of the hardware.

Since the driver does not use separate queues for mini or jumbo frames,
attempts to set those values are rejected.

If the interface is already running when the setting is changed, then
the interface is reset.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>