usb: musb: Fix getting a generic phy for musb_dsps

We still have a combination of legacy phys and generic phys in
use so we need to support both types of phy for musb_dsps.c.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>