net: designware: use 'phy_connect' instead of open coded

Using 'phy_connect' instead of 'phy_find_by_mask' and 'phy_connect_dev'
both deduplicates code and adds support for 'fixed-link'.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>