mfd: omap-usb-host: get rid of build warning

Fixes the below build warning when driver is built-in.

drivers/mfd/omap-usb-host.c:750:12: warning:
‘usbhs_omap_remove’ defined but not used [-Wunused-function]

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>