USB: fsl-mph-dr-of: adjust for OF based clock lookup

after device tree based clock lookup became available, the peripheral
driver need no longer construct clock names which include the component
index -- remove the "usb%d_clk" template, always use "ipg" instead

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>