usb: cdns3: cdns3-ti: Fix clk_get_by_name() to get the correct name

Kernel device tree got updated to use clock name as "ref" instead of
"usb2_refclk". Fix cdns3-ti.c to use the correct name.

Fixes: 70e16749 ("arm: dts: k3-j721e: Sync Linux v5.11-rc6 dts into U-Boot")
Fixes: 6239cc8c ("arm: dts: k3-j7200: Sync Linux v5.11-rc6 dts into U-Boot")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>