phy: omap-usb2: invoke pm_runtime_disable on error path

if devm_clk_get for wkupclk fails, there will be an unbalanced
pm_runtime_enable. Fix it here.

Reported-by: Benoit Parrot <bparrot@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>