cbus-tahvo-usb: Initialize the used clock

usb_l4_ick is used by the USB. Request and enable it.
Without this, all register accesses just return garbage
due to the device not being clocked.

This also cleans up the error path of the probe function
while we're at it.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>