net: stmmac: optimization and updation of phydev

Addition of parameter phydev in stmmac data structure to
avoid costly operation of fetching the phydev, fetch phydev and
store the phydev during mdio register.
Fetching the phydev from stmmac priv as and when required.

Change-Id: I7fe0c81cdc2b8ef12f2292029bad260618430763
Acked-by: Abhishek Chauhan <abchauha@qti.qualcomm.com>
Signed-off-by: Sunil Paidimarri <hisunil@codeaurora.org>