usb: musb: ux500: Don't supply a con_id when requesting the clock

If we supply a con_id then the clock framework will search for that name
in MUSB's Device Tree node for the 'clock-names' property. If it's absent
the clock request will fail. However, if we don't supply the con_id then
clk_get() will call into clk_sys() which will use the device name to
search for the appropriate clock, which is much more natural than forcing
'usb'.

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>