usb/misc/usb3503: Always read refclk frequency from DT

This is necessary to set REF_SEL appropriately in uses where refclk is
always available.

Signed-off-by: Ben Gamari <ben@smart-cactus.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>