usb: musb: ux500: use clk_prepare_enable and clk_disable_unprepare

This patch converts the module to use clk_prepare_enable and
clk_disable_unprepare variants as required by common clock framework.

Without this the system crash during probe function.

Cc: <stable@vger.kernel.org> # v3.7 v3.8
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>