[realtek] Work around hardware bug on RTL8211B

[realtek] Work around hardware bug on RTL8211B

The RTL8211B seems to have a bug that prevents the link from coming up
unless the MII_MMD_DATA register is cleared.

The Linux kernel driver applies this workaround (in rtl8211b_resume())
only to the specific RTL8211B PHY model, along with a matching
workaround to set bit 9 of MII_MMD_DATA when suspending the PHY.
Since we have no need to ever suspend the PHY, and since writing a
zero ought to be harmless, we just clear the register unconditionally.

Debugged-by: Nikolay Pertsev <nikolay.p@cos.flag.org>
Tested-by: Nikolay Pertsev <nikolay.p@cos.flag.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>