bnxt_en: Add support for mdio read/write to external PHY

Add support for SIOCGMIIREG and SIOCSMIIREG ioctls to
mdio read/write to external PHY.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>