mfd: omap-usb: prepare/unprepare clock while enable/disable

This should fix the following warning at boot on OMAP5 uEVM
[    8.783155] WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:883 __clk_enable+0x94/0xa4()

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>