mfd: Fix the omap-usb-host clock API usage on usbhs_disable()

mfd: Fix the omap-usb-host clock API usage on usbhs_disable()

usbhs_disable function was invoking clk_enable() instead of
clk_disable(), thus only increasing the clock usage counter and
preventing this particular clock from being ever turned off.
Because of this, the power domain of omap4 the USB Host subsystem
would never reach lower power states.This patch calls clk_disable()
in usbhs_disable function

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>