phy: lantiq: Fix compile warning

This local variable is unused, remove it.

Fixes: dea54fba ("phy: Add an USB PHY driver for the Lantiq SoCs using the RCU module")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>