usb: phy: am335x-control: check return value of bus_find_device

This fixes a potential null pointer dereference.

Cc: <stable@vger.kernel.org> # v3.16+
Fixes: d4332013 ("driver core: dev_get_drvdata: Don't check for NULL dev")
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David Dueck <davidcdueck@googlemail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>