net: Add mii_resolve_flowctrl_fdx()

Add an mii helper function to resolve flow control status per
IEEE 802.3-2005 table 28B-3.
This function was taken from the Linux source tree.

Signed-off-by: Yuiko Oshino <yuiko.oshino@microchip.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>