phy: omap-usb2: Fix missing clk_prepare call when using old dt name

Current code does not call clk_prepare(phy->optclk) when using the old
usb_otg_ss_refclk960m name. Fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>