Fix the way the local advertising flow options are determined

Local flow control options needed in order to resolve the negotiation
are incorrectly calculated.

Previously 'mii_advertise_flowctrl' was called to determine the local advertising
options, but these were determined based on FLOW_CTRL_RX/TX flags which are
never set through ethtool.
The patch simply translates from ethtool flow options to mii flow options.

Signed-off-by: Pavaluca Matei <matei.pavaluca@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>