usb: phy: return an error in usb_get_phy() if try_module_get() fails

In case we found a matching USB PHY in usb_get_phy() but the call to
try_module_get() fails, we shouldn't return a (probably soon dangling)
pointer but an ERR_PTR instead.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>