usb: of: fix build breakage on !OF

If OF is disabled, we will try to define a stub for
of_usb_get_dr_mode_by_phy(), however that missed a
static inline annotation which made us redefine the
stub over and over again. Fix that.

Fixes: 98bfb394 ("usb: of: add an api to get
	dr_mode by the phy node")
Signed-off-by: Felipe Balbi <balbi@ti.com>