Per the reports, some Allwinner device trees now list the desired
phy-mode as "rgmii-id". The manual string comparison fails to detect
this, and we end up falling back to MII mode. Instead, select the clock
name using the sc->phy_mode variable, which is set in the main attach
function.

The logic to actually handle rgmii-id mode delays will be added to the
relevant PHY driver.

PR:		261355, 264673
Reported by:	Maren <marentoy@protonmail.com>
Reported by:	Arie Bikker <src-2016@bikker.homeunix.net>
Reviewed by:	manu
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35500