usb: gadget: mv_udc: fix the clk APIs

the clock common driver changes, and arch-mmp will make use of
the common clock driver instead of its own.
So for enable clock.
first prepare the clock
then enable the clock.

for disable clock
first disable the clock
then unprepare the clock

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>