tg3 / broadcom: Add PHY_BRCM_CLEAR_RGMII_MODE flag

Broadcom 50610M parts changed the default definitions of the RGMII mode
shadow register.  The 5785 needs the RGMII mode selection bits [4:3]
cleared.

The default value of the remaining bits in this register are zero.
Rather than unnecessarily burn an extra bit in the dev_flags member in
an attempt to enumerate all possible combinations, this patch take a
more course grained approach and labels the option as "clear all bits".

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>