usb: r8a66597-hcd: Convert to clk_prepare/unprepare

Turn clk_enable() and clk_disable() calls into clk_prepare_enable() and
clk_disable_unprepare() to get ready for the migration to the common
clock framework.

Cc: linux-usb@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>