net: phy: dp83867: Clean force link good bit

On Xilinx ZynqMP revA board initial value of PHYCR register is 0x5448 which
means FORCE_LINK_GOOD is already setup. Origin code was doing write but the
new code is doing read/modify/write and keep this bit untouched. That's why
ethernet stop to work.
The patch is cleaning this bit when PHYCR value is composed.

Tested on Xilinx zcu102-revA and zcu104-rev1.0 boards.

Fixes: 37d6265f ("net: phy: dp83867: refactor rgmii configuration")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>